Ch 2 · Light: Reflection, Refraction, Lenses and Dispersion · IGCSE Physics 0625
Topic 2 of 3 in Waves — IGCSE Physics 0625 — 50 lessons.
Introduction
Mirrors, spectacles, cameras, optical fibres and rainbows all depend on light changing direction — by reflection at a surface or by refraction as it changes speed. This lesson covers the laws of reflection and refraction, total internal reflection, ray diagrams for lenses and the splitting of white light into colours. Ray diagrams must be drawn with a ruler and sharp pencil; many marks are lost for freehand or unlabelled rays.
Key ideas — Reflection
1. The law of reflection. The normal is a line drawn at 90^° to the surface where the ray hits it. The angle of incidence i and angle of reflection r are both measured from the normal, and
🧑🏫 Teacher's explanation
Imagine you are standing in front of a mirror. When you raise your right hand, your reflection raises its left hand—this is 'lateral inversion'. When light hits a smooth surface like a mirror, it doesn't scatter in all directions; instead, it bounces off at the exact same angle it arrived.
📖 Definitions
🔍 Worked example — Easy
A ray of light hits a plane mirror at an angle of 35^° to the mirror surface. Calculate the angle of reflection.
🔍 Worked example — Standard
An object is placed 12 ext cm in front of a plane mirror. Determine the distance between the object and its image.
🔍 Worked example — Exam-style
A ray of light is incident on a plane mirror. The angle between the incident ray and the reflected ray is 80^°. Calculate the angle of incidence.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) State the distance from the image to the mirror. [1] (b) The student moves the object to a distance of 15.0 cm from the mirror. Calculate the new distance between the object and its image. [2] [3 marks]
Key ideas — Refraction
3. Refraction is the change in direction of light when it crosses a boundary at an angle, because its speed changes.
🧑🏫 Teacher's explanation
Imagine you are running from a paved sidewalk into a thick patch of sand at an angle. As your lead foot hits the sand, it slows down immediately, causing your body to pivot or 'bend' towards the direction of the sand. This is exactly what light does when it moves from air into glass. Because light travels at different speeds in different materials, its path changes whenever it hits a boundary at an angle.
📖 Definitions
🔍 Worked example — Easy
A ray of light travels from air into a glass block. The angle of incidence is 30^° and the refractive index of the glass is 1.50. Calculate the angle of refraction.
🔍 Worked example — Standard
Calculate the critical angle for a glass-air boundary where the refractive index of glass is 1.52.
🔍 Worked example — Exam-style
Light travels from a liquid into air. The critical angle is 45.0^°. Calculate the refractive index of the liquid.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Key ideas — Thin lenses
7. Ray diagram rules for a converging lens. Use any two of:
🧑🏫 Teacher's explanation
Think of a lens as a tool that bends light rays to change where they meet. A converging lens is like a magnifying glass; it takes parallel rays and bends them inward toward a single point. A diverging lens does the opposite, spreading light rays apart.
📖 Definitions
🔍 Worked example — Easy
A converging lens has a focal length of 10 cm. An object is placed 30 cm from the lens. Determine if the image is real or virtual.
🔍 Worked example — Standard
An object is placed 5.0 cm from a converging lens with a focal length of 10 cm. Describe the characteristics of the image formed.
🔍 Worked example — Exam-style
A student uses a converging lens to project an image onto a screen. The object is 15 cm from the lens and the image is formed 30 cm on the other side. Calculate the focal length f of the lens.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Draw a ray diagram to show the formation of the image. (b) State the characteristics of the image formed (size, orientation, and type). [4 marks]
Key ideas — Dispersion
8. Dispersion. A glass prism splits white light into a spectrum because each colour is refracted by a slightly different amount (violet most, red least). The seven traditional colours, in order of increasing frequency and decreasing wavelength, are:
🧑🏫 Teacher's explanation
Imagine a group of runners all sprinting at the same speed, but they are running through a patch of thick mud. If the runners are different sizes, the smaller ones might struggle more and slow down more than the larger ones. Light behaves similarly when it enters a prism. White light is actually a mixture of many different colors, each traveling at a slightly different speed when it enters glass.
📖 Definitions
🔍 Worked example — Easy
List the seven colors of the visible spectrum in order of increasing frequency.
🔍 Worked example — Standard
A beam of monochromatic light is passed through a glass prism. Describe what happens to the beam.
🔍 Worked example — Exam-style
In a prism, violet light is refracted more than red light. If red light has a longer wavelength than violet light, which color has a higher frequency?
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Describe the effect of the prism on the white light. [2] (b) State the order of the colors in the spectrum produced, starting from the color that is refracted the least. [1] [3 marks]
Practical notes
Worked example
Example 1 — plane mirror. A pupil stands 1.5 m in front of a plane mirror. How far is she from her image? The image is 1.5 m behind the mirror, so the distance between pupil and image is 1.5 + 1.5 = 3.0\ m.
🔍 Worked example — Easy
A ray of light travels from air into a glass block. The angle of incidence is 35^° and the angle of refraction is 22^°. Calculate the refractive index n of the glass.
🔍 Worked example — Standard
Light travels from a diamond (n = 2.42) into air. Calculate the critical angle c for the diamond-air boundary.
🔍 Worked example — Exam-style
An object of height 5.0 cm is placed 15.0 cm from a converging lens with a focal length of 10.0 cm. Calculate the height of the image formed.
⚠️ Common errors
Common mistakes and examiner tips
Practice
Answers: (1) The angle of incidence is 90 - 35 = 55^°, so the angle of reflection is 55^°. (2) Light must travel from a denser to a less dense medium, and the angle of incidence must be greater than the critical angle. (3) Any two: carry more data (higher rate); less signal loss over long distances; not affected by electrical interference; more secure; lighter and thinner. (4) Real, inverted, enlarged, beyond 2F on the other side. (5) Violet, indigo, blue, green, yellow, orange, red. (6) sin r = sin 40^°/1.33 = 0.643/1.33 = 0.483, so r = 29^°. (7) sin c = 1/2.42 = 0.413, so c = 24^°. (8) v = 3.0 × 10^8/1.5 = 2.0 × 10^8\ m/s. (9) Short sight; a diverging (concave) lens.