Light

O Level / IGCSE Physics · Waves. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.

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The idea

Light travels in straight lines called rays. Every angle is measured from the normal, a line drawn at 90° to the surface where the ray meets it.

A mirror reflects light so that the angle of reflection equals the angle of incidence. When light enters a different transparent material its speed changes and it bends. This is refraction. Going into glass or water the ray bends towards the normal; coming out it bends away. If the ray inside the glass meets the surface at more than the critical angle, none of it escapes. This is total internal reflection, used in optical fibres.

A converging lens brings parallel rays to a point, the principal focus. A diverging lens spreads them out.

Rules to remember

Reflection: angle of incidence = angle of reflection (i = r). A plane mirror image is virtual, upright, the same size as the object, and as far behind the mirror as the object is in front.
refractive index = sin i ÷ sin r (n = sin i/sin r), where i is the angle in air. It has no unit.
refractive index = 1 ÷ sin c (n = 1/sin c), where c is the critical angle.
linear magnification = image length ÷ object length. It has no unit.
Converging lens: an object beyond the principal focus gives a real, inverted image. An object closer than the principal focus gives a virtual, upright, magnified image (magnifying glass).
Short sight: image forms in front of the retina, corrected by a diverging lens. Long sight: image would form behind the retina, corrected by a converging lens.
Spectrum in order of increasing frequency: red, orange, yellow, green, blue, indigo, violet. Red has the longest wavelength; violet is refracted the most.

Common mistake

Students measure angles from the mirror or glass surface. Angles of incidence, reflection and refraction are always measured from the normal, so subtract from 90° if the question gives the angle to the surface.

Worked example

A ray of light in air meets a glass block at an angle of incidence of 30°. The refractive index of the glass is 1.5. What is the angle of refraction? (sin 30° = 0.50, sin 19.5° = 0.333)

n = sin i ÷ sin r, so sin r = sin i ÷ n.
sin r = 0.50 ÷ 1.5 = 0.333.
The angle whose sine is 0.333 is 19.5°.
Check: the ray enters glass, so it bends towards the normal and r is less than i. It is.

Answer: About 19.5°

Practice questions

Try each one, then open the answer.

1. A ray of light strikes a plane mirror at an angle of incidence of 35°. What is the angle between the reflected ray and the mirror surface?

A
35°
B
45°
C
55°
D
70°

Show answer

Answer: C. Angle of reflection = angle of incidence = 35°, measured from the normal. From the surface it is 90° − 35° = 55°.

2. An object is placed between a converging lens and its principal focus. The image is

A
real, inverted and diminished
B
real, inverted and magnified
C
virtual, upright and magnified
D
virtual, inverted and the same size

Show answer

Answer: C. Inside the focal length the lens acts as a magnifying glass: the rays diverge after the lens and the image is virtual, upright and enlarged, on the same side as the object.

3. An object 2.0 cm tall forms an image 6.0 cm tall. What is the linear magnification?

A
0.33
B
3.0
C
4.0
D
12

Show answer

Answer: B. linear magnification = image length ÷ object length = 6.0 ÷ 2.0 = 3.0 (no unit).

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