Light: Complete O Level Physics Cheatsheet
O Level Physics, Chapter 14 · Read time: ~10 minutes
These notes come from TGC Academy's O Level Physics tuition in Singapore.
Key Formulas
Reflection
Law of reflection: the angle of incidence equals the angle of reflection. Both are measured from the normal, a line perpendicular to the surface at the point where the ray hits.
An image in a plane mirror is the same size as the object, upright, laterally inverted, as far behind the mirror as the object is in front, and virtual (it cannot be caught on a screen).
Trap: Angles are always measured from the normal, never from the mirror surface. Draw the normal as a dashed line first, then mark the angles.
Refraction and refractive index
Refraction is the bending of light when it enters a different medium, because its speed changes.
Light going from air into glass slows down and bends towards the normal. Light going from glass into air speeds up and bends away from the normal.
Refractive index (n) = speed of light in vacuum ÷ speed of light in the medium, n = c/v. It also equals sin i / sin r. The higher n is, the more the light slows down and bends.
Critical angle and total internal reflection
Critical angle (c): the angle of incidence in the denser medium for which the angle of refraction is 90°. sin c = 1/n.
Total internal reflection happens when light travels from a denser medium towards a less dense one and the angle of incidence is greater than the critical angle. All the light reflects back inside.
Optical fibres use total internal reflection to carry light along a bent glass fibre. In telecommunications they carry more data than copper wires, with less signal loss and no electrical interference. In medicine they are used in endoscopes to see inside the body without major surgery.
Trap: Total internal reflection can never happen when light goes from air into glass. The light must already be inside the denser medium, trying to get out.
Thin converging lenses
A converging (convex) lens refracts parallel rays of light so that they meet at the principal focus (F). The focal length (f) is the distance from the centre of the lens to F.
Ray diagram rules: a ray parallel to the principal axis passes through F after the lens. A ray through the centre of the lens passes straight through. A ray through F on the near side comes out parallel to the axis.
Object beyond 2F: real, inverted, diminished image (camera). Object at 2F: real, inverted, same size. Object between F and 2F: real, inverted, magnified (projector). Object between F and the lens: virtual, upright, magnified (magnifying glass).
A real image forms where rays actually meet and can be shown on a screen. A virtual image is where rays only appear to come from.
Trick: Draw any two of the three standard rays. Where they cross is the top of the image. If they spread apart after the lens, extend them backwards as dashed lines: the image is virtual and on the same side as the object.
Worked Example
A ray of light in air strikes a glass block at an angle of incidence of 40° and refracts at 25° inside the glass.
(a) Calculate the refractive index of the glass. [2]
n = sin i / sin r
= sin 40° / sin 25°
= 0.643 / 0.423
= 1.52
(b) Calculate the critical angle of the glass. [2]
sin c = 1 / n = 1 / 1.52 = 0.658
c = 41.1°
(c) Inside the block, the ray later hits a glass-air surface at an angle of incidence of 50°. State and explain what happens. [2]
The ray is totally internally reflected.
It is travelling from glass (denser) towards air (less dense), and 50° is greater than the critical angle of 41.1°.
Why this question is a good test of the topic: it chains three skills, finding n from angles, using n to find the critical angle, then checking both conditions for total internal reflection. One slip early on breaks the rest of the chain.
Frequently Asked Questions
What two conditions are needed for total internal reflection?
Light must travel from a denser to a less dense medium, and the angle of incidence must be greater than the critical angle.
What is refractive index?
The speed of light in vacuum divided by the speed of light in the medium. It also equals sin i / sin r.
Why are optical fibres used in telecommunications?
They carry more data than copper cables, lose less signal over long distances and are not affected by electrical interference.
What is the focal length of a converging lens?
The distance from the centre of the lens to the principal focus, where parallel rays meet after passing through the lens.
When does a converging lens form a virtual image?
When the object is between the principal focus and the lens. The image is virtual, upright and magnified, as in a magnifying glass.
Ray diagrams still not clicking?
Small group O Level Physics classes at TGC Academy's Bishan, Bukit Timah and Potong Pasir centres, helmed by Andrew Seah, MOE Award-Winning Teacher and Marshall Cavendish textbook author.