✨ Detailed Explanation of Reflection and Refraction of Light
Reflection and refraction of light are fundamental concepts in Year 10 Physics that explain how light behaves when it encounters different surfaces or materials. Understanding these phenomena helps us learn about how light changes direction and speed, which is essential for many real-life applications.
🔍 Reflection of Light
Reflection occurs when light rays bounce off a surface. The surface can be smooth, like a mirror, or rough, like a wall. There are two types of reflection:
- Specular reflection: Light reflects at a single angle, producing clear images (like in mirrors).
- Diffuse reflection: Light scatters in many directions, so no clear image is formed.
📏 Laws of Reflection
There are two important laws that govern reflection:
- The angle of incidence equals the angle of reflection.
This means the angle at which light hits the surface (angle of incidence) is exactly the same as the angle at which it bounces off (angle of reflection). Both angles are measured from the normal line, which is a line perpendicular (at 90°) to the surface at the point of incidence. - The incident ray, reflected ray, and normal all lie in the same plane.
Diagram of Reflection:
Reflected ray
/
/ θr (angle of reflection)
Normal |
| |
| |
------------ Surface (mirror)
| |
| |
\ θi (angle of incidence)
\
Incident ray
🌈 Refraction of Light
Refraction is the bending of light as it passes from one transparent material to another with a different density (e.g., from air to water or glass). This change occurs because the speed of light changes when moving between different materials.
⚡ Effects on Light Direction and Speed
- When light enters a denser medium (like water or glass) from a less dense medium (like air), it slows down and bends towards the normal.
- When light passes from a denser medium to a less dense medium, it speeds up and bends away from the normal.
The amount of bending depends on the refractive indices of the two materials.
📐 Laws of Refraction (Snell’s Law)
- The incident ray, refracted ray, and the normal all lie in the same plane.
- The relationship between the angle of incidence (θ₁) and the angle of refraction (θ₂) is given by Snell’s Law:
n₁ sin θ₁ = n₂ sin θ₂
where n₁ and n₂ are the refractive indices of the first and second medium respectively.
Diagram of Refraction:
Refracted ray
/
/ θr (angle of refraction)
Normal |
| |
--------- Boundary (air to glass)
| |
| |
\ θi (angle of incidence)
\
Incident ray
🔧 Applications in Everyday Life
- Reflection: Mirrors, periscopes, and car headlights use reflection. A smooth reflection makes clear images, while diffuse reflection helps us see walls and other objects.
- Refraction: Lenses in glasses, cameras, and microscopes depend on refraction to bend light and focus it properly. Refraction also explains why objects appear bent or broken in water.
📝 Summary
Reflection and refraction are important for understanding how light behaves when interacting with different surfaces and materials. By applying the laws of reflection and refraction, we can predict and explain many optical effects we see every day.
📚 Study Tip: Try drawing diagrams of reflection and refraction yourself with different angles. Label the normal, incident ray, reflected or refracted ray, and measure the angles to see how they follow the laws. This will help you visualise the concepts better for exams!
🔢 10 Examination-style 1-Mark Questions with 1-Word Answers on Reflection and Refraction
- What is the angle called between the incident ray and the normal in reflection?
Answer: Angle of incidence - What is the term for light bending as it passes from one medium to another?
Answer: Refraction - What do we call the surface from which light is reflected?
Answer: Mirror - What is the name of the ray that strikes a reflective surface?
Answer: Incident - What name is given to the ray that leaves a reflective surface?
Answer: Reflected - When light passes from air into water, does it bend towards or away from the normal?
Answer: Towards - What is the point called where the incident ray meets the reflective surface?
Answer: Point of incidence - What is the term for a surface that causes scattering of light rays?
Answer: Rough - Which law states that the angle of incidence equals the angle of reflection?
Answer: Reflection - What type of lens causes light rays to converge?
Answer: Convex
📝 10 Examination-style 2-Mark Questions with 1-Sentence Answers on Reflection and Refraction
1. What is the law of reflection?
Answer: The angle of incidence is equal to the angle of reflection when light reflects off a surface.
2. What happens to light when it passes from air into water?
Answer: The light ray slows down and bends towards the normal due to refraction.
3. Define the term ‘normal’ in reflection and refraction.
Answer: The normal is an imaginary line perpendicular to the surface where the light ray meets it.
4. What is total internal reflection?
Answer: It is when light reflects entirely inside a denser medium instead of refracting out into a less dense medium.
5. Why does a pencil look bent when partly submerged in water?
Answer: Because light bends as it moves between air and water, causing the pencil to appear angled.
6. What type of lens bends light rays to converge at a point?
Answer: A convex lens bends light rays to a focal point by converging them.
7. How does the speed of light change when it moves from glass to air?
Answer: The speed of light increases as it moves from glass (denser) into air (less dense).
8. What is the refractive index of a material?
Answer: It is the ratio of the speed of light in a vacuum to the speed of light in that material.
9. How is the angle of refraction related to the angle of incidence?
Answer: The angle of refraction depends on the refractive indices of the two media and usually differs from the angle of incidence.
10. Why can mirrors form clear images by reflection?
Answer: Because they produce regular reflection where parallel light rays reflect at the same angle, forming clear images.
📝 10 Examination-style 4-Mark Questions with 6-Sentence Answers on Reflection and Refraction
Question 1
Explain what happens when light is reflected from a smooth, plane mirror.
Answer:
When light hits a smooth plane mirror, it bounces off the surface in a specific way called reflection. The angle at which the light strikes the mirror, called the angle of incidence, is equal to the angle at which it reflects away, known as the angle of reflection. This is called the law of reflection. The surface of the mirror must be smooth to produce a clear image. If the surface were rough, the light would scatter in different directions, creating a diffused reflection. Therefore, plane mirrors produce clear, sharp images by reflecting light at equal angles.
Question 2
Describe what refraction is and give one everyday example.
Answer:
Refraction is the bending of light as it passes from one transparent medium to another where its speed changes. This bending happens because light travels at different speeds in materials like air, water, or glass. For example, when a straw is placed in a glass of water, it looks bent or broken at the surface of the water. This occurs because light slows down when entering water from air and changes direction. Refraction is important for lenses used in glasses and cameras. It helps focus light to form clear images.
Question 3
State the laws of reflection and explain why the angle of incidence equals the angle of reflection.
Answer:
The first law of reflection states that the incident ray, the reflected ray, and the normal (a line perpendicular to the surface) all lie in the same plane. The second law states that the angle of incidence is equal to the angle of reflection. This happens because light reflects off a smooth surface in a regular and predictable way. The equality of the angles ensures that the path of the reflected light is symmetrical to the incident light. This symmetry is why mirrors can produce clear images. Without these laws, reflections would be unpredictable and blurry.
Question 4
Explain how refraction causes a change in the speed of light and why light bends towards the normal when it enters a denser medium.
Answer:
When light passes from a less dense medium like air into a denser medium like water, it slows down because the particles are closer together. This change in speed causes the light to change direction, resulting in bending, or refraction. Light bends towards the normal because it slows down upon entering the denser medium. The normal is an imaginary line perpendicular to the surface at the point of incidence. This bending is why objects under water appear shifted or closer than they actually are. The speed difference and bending follow Snell’s law, which relates angles and refractive indices.
Question 5
A ray of light strikes a glass surface at an angle of 30°. Draw and describe the path of the ray as it reflects and refracts.
Answer:
When a ray of light hits a glass surface at 30°, part of the light reflects off the surface, following the law of reflection where the angle of reflection equals 30°. The reflected ray bounces off at this same angle away from the normal line. Meanwhile, some of the light passes into the glass and refracts. Because glass is denser than air, the refracted ray bends towards the normal line inside the glass. The angle of refraction is smaller than 30° due to the change in speed. This combined reflection and refraction explain why light splits at boundaries between different materials.
Question 6
What causes total internal reflection and in what situation does it occur?
Answer:
Total internal reflection occurs when light tries to pass from a denser medium to a less dense medium at an angle greater than the critical angle. At this critical angle, the refracted light would travel along the boundary between the two media. If the angle of incidence is larger, the light instead reflects back entirely into the denser medium. No refraction occurs in this case. This phenomenon happens, for example, in optical fibres, where light is trapped inside by total internal reflection. It is useful for transmitting signals over long distances without loss.
Question 7
How does the refractive index of a material relate to the speed of light in that material?
Answer:
The refractive index of a material tells us how much slower light travels in that material compared to in a vacuum. It is calculated by dividing the speed of light in a vacuum by the speed of light in the material. A higher refractive index means that light travels more slowly. For example, glass has a higher refractive index than air, so light slows down more in glass. This slowing down causes the light ray to bend when passing between different materials. Knowing the refractive index helps predict the behavior of light during refraction.
Question 8
Describe what happens to light when it passes from air into water at an angle other than 90°.
Answer:
When light passes from air into water at an angle other than 90°, it slows down because water is denser than air. This change in speed causes the light ray to bend towards the normal line at the surface. The angle of refraction inside the water is smaller than the angle of incidence in air. Some light also reflects off the water surface, following the law of reflection. The bending of light is something you can observe when looking at a stick partially submerged in water. This is a clear example of refraction in everyday life.
Question 9
Explain why a pencil appears bent when placed in a glass of water.
Answer:
A pencil appears bent in water because of refraction. Light rays travel from the pencil through water and then into the air before reaching your eyes. When light passes from water (denser medium) to air (less dense medium), it speeds up and bends away from the normal. This bending causes the pencil to seem broken or bent at the water’s surface. Your brain interprets the light as if it travelled in a straight line, creating a distorted image. This visual effect clearly demonstrates how refraction works.
Question 10
How does the angle of incidence affect the amount of light that is reflected and refracted at a boundary?
Answer:
The angle of incidence influences the proportion of light that reflects or refracts at a boundary between two media. At small angles, most light usually refracts into the second medium with little reflection. As the angle of incidence increases, more light reflects off the surface and less refracts. At very high angles, near the critical angle, almost all light can reflect, causing total internal reflection. This relationship is described by Fresnel’s equations. Understanding this helps in designing devices like lenses, glasses, and optical fibres.
📝 10 Examination-style 6-Mark Questions with 10-Sentence Answers on Reflection and Refraction
Question 1
Explain what happens to a light ray when it strikes a plane mirror at an angle, describing the laws of reflection.
Answer:
When a light ray hits a plane mirror at an angle, it reflects off the surface. The angle at which the light ray arrives is called the angle of incidence. According to the law of reflection, the angle of incidence equals the angle of reflection. Both angles are measured from the normal, which is an imaginary line perpendicular to the surface at the point of incidence. The reflected ray stays in the same plane as the incident ray and the normal. This means if the light ray hits the mirror at 30 degrees to the normal, it will reflect away also at 30 degrees. This behaviour helps to explain how we see images in mirrors. Plane mirrors produce virtual images that appear the same distance behind the mirror as the object in front. These images are laterally inverted, meaning left and right are swapped. This understanding is important in many optical instruments and daily life.
Question 2
Describe the refraction of light as it passes from air into glass and explain why the light ray bends.
Answer:
When light travels from air into glass, it slows down because glass is denser than air. This change in speed causes the light to bend, a process called refraction. The light ray bends towards the normal line because it slows down. The normal is an imaginary line perpendicular to the surface where the light enters the glass. The amount the light bends depends on the angle of incidence and the optical density of the materials. Glass has a higher refractive index than air, which measures how much light slows down in a material. The bending of light is described by Snell’s Law, which relates the angles and refractive indices of both materials. As light exits the glass back into air, it speeds up and bends away from the normal. This bending of light is responsible for effects like a straw looking bent in a glass of water. Refraction is used in lenses to focus light in glasses and cameras.
Question 3
What is total internal reflection, and under what conditions does it occur?
Answer:
Total internal reflection happens when light tries to move from a denser material to a less dense material but cannot pass through the boundary. Instead, all the light reflects back into the denser material. This occurs only if the angle of incidence inside the denser material is greater than the critical angle specific for that pair of materials. The critical angle is the smallest angle of incidence where total internal reflection begins. Total internal reflection can only happen when light is travelling from a material with a higher refractive index to one with a lower refractive index. For example, from glass to air. If the incident angle is less than the critical angle, some light refracts out of the glass. Total internal reflection is important in fibre optics, where light signals are kept inside thin glass fibres. This process allows fast and efficient transmission of information over long distances. It is also used in some optical instruments, like binoculars and prisms.
Question 4
Explain how a convex lens refracts light and how it can form real and virtual images.
Answer:
A convex lens is thicker in the middle and thinner at the edges, which causes light rays to converge or come together. When parallel rays of light pass through a convex lens, they refract towards the principal focus, the point where the rays meet. If an object is placed beyond the focal length of the lens, it forms a real image on the other side of the lens. This real image can be projected onto a screen and is inverted, meaning it is upside down compared to the object. If the object is inside the focal length, the rays diverge after passing through the lens, and the eye traces these rays backward to form a virtual image. This virtual image appears upright and larger and cannot be projected. Convex lenses are used in magnifying glasses, cameras, and the human eye to focus light. The bending of light by the lens is explained by refraction, where light changes speed and direction when entering and leaving the lens material. The shape and material of the lens affect how strongly it bends light.
Question 5
How does the refractive index of a material affect the speed of light and the refraction angle?
Answer:
The refractive index of a material measures how much it slows down light compared to vacuum. A higher refractive index means light travels slower in the material. When light passes from one material to another, such as air to glass, the change in speed causes the light to bend. This bending is called refraction. The refractive index determines the ratio of sine of the angle of incidence to sine of the angle of refraction, known as Snell’s Law. If the refractive index is large, the light bends more towards the normal when entering the material. If it is smaller or closer to 1, light bends less. This is why light bends more when passing into glass or water compared to air. The slower the speed of light in the new material, the greater the refraction angle change. Understanding refractive index helps design lenses and optical devices. It also explains natural phenomena like the apparent bending of objects under water.
Question 6
Describe how light behaves when passing through a triangular glass prism and why dispersion occurs.
Answer:
When white light enters a triangular glass prism, it slows down and bends towards the normal due to refraction. As light exits the prism, it speeds up and bends away from the normal. The prism separates the white light into its different colours because each colour has a slightly different wavelength. Different wavelengths of light refract at different angles, causing dispersion. Blue light bends more than red light because it travels slower in glass. The triangular shape of the prism helps the separated colours spread out to form a spectrum. This spectrum shows all the colours of visible light, from red to violet. Dispersion explains why rainbows form when sunlight passes through water droplets. The bending and separation of light by a prism are important in studying light’s properties. Understanding this helps in designing instruments like spectrometers.
Question 7
Explain why a spoon in a glass of water appears bent at the surface.
Answer:
A spoon appears bent at the water surface because of refraction. Light rays travel from the spoon underwater to the air and change speed as they move from water to air. Water is denser than air, so light speeds up when leaving the water. This change in speed causes the light rays to bend away from the normal as they leave the water. Our eyes trace these rays back in a straight line, making the spoon look bent or broken at the surface. The angle of refraction changes the apparent position of the spoon. The bending effect creates an optical illusion of displacement. This shows how light paths are altered when moving between media with different densities. This is a common example used to understand refraction in real life. It also explains why objects under water look closer than they really are.
Question 8
What is the critical angle, and how is it related to total internal reflection?
Answer:
The critical angle is the angle of incidence in the denser material at which refracted light just grazes along the boundary surface between two materials. It is the minimum angle where the angle of refraction is 90 degrees. When light hits the interface at an angle greater than the critical angle, it does not refract out but instead totally internally reflects. Total internal reflection occurs only when light travels from a denser to a less dense medium, like from glass to air. The critical angle depends on the refractive indices of the two materials. If the angle of incidence is less than the critical angle, refraction occurs, and light passes through. If it is equal to the critical angle, the refracted ray travels along the boundary. This principle is used in fibre optics to keep light trapped inside the cable. Understanding critical angle helps design optical fibres and devices that rely on internal reflection.
Question 9
How do plane mirrors produce images, and what are the characteristics of these images?
Answer:
Plane mirrors produce images by reflecting light rays according to the law of reflection. The image appears behind the mirror at the same distance as the object is in front. This image is virtual because light rays do not actually come from behind the mirror. The image cannot be projected on a screen. The image is the same size as the object and laterally inverted, meaning left and right are reversed. The image is upright, maintaining the object’s orientation. Light rays hitting the mirror follow the rule that angle of incidence equals angle of reflection. Plane mirrors do not converge or diverge light but just change its direction. This explains why we see clear, undistorted reflections. These properties make plane mirrors useful for everyday purposes like shaving and dressing.
Question 10
Explain how refraction allows lenses to correct vision problems like short-sightedness or long-sightedness.
Answer:
Refraction bends light as it passes through lenses, allowing correction of vision problems. Short-sighted people cannot see distant objects clearly because their eyes focus images in front of the retina. Concave lenses diverge light rays before entering the eye, moving the focus back onto the retina. Long-sighted people cannot see nearby objects clearly because their eyes focus images behind the retina. Convex lenses converge light rays so the focus moves forward onto the retina. By adjusting where the light focuses, lenses compensate for the eye’s shape flaws. Refraction inside the lenses changes the light direction to correct vision. Understanding how light bends helps opticians prescribe lenses. Without refraction through lenses, vision problems would not be easily corrected. This application shows the practical importance of refraction in daily life.
