The law of reflection

When light hits a plane (flat) mirror it bounces off. Angles are always measured from the normal — an imaginary line drawn at right angles to the mirror at the point where the ray hits.

Watch out: angles are measured from the normal, not from the surface of the mirror. A ray hitting at 30° to the normal reflects at 30° to the normal.

Your turn — angle of reflection

The image in a plane mirror

Trace the reflected rays backwards and they appear to come from a point behind the mirror. The image you see there is:

• Virtual — no light actually reaches it, so it can't be caught on a screen;
• Upright — the same way up as the object;
• The same size as the object;
• As far behind the mirror as the object is in front;
• Laterally inverted — left and right are swapped (which is why AMBULANCE is written back-to-front on the bonnet).

Reading the mirror

Refraction: a change of speed

When light crosses a boundary between two materials, its speed changes. Because of this, the ray usually changes direction — this is refraction.

• Going into a denser material (air → glass) light slows down and bends towards the normal.
• Coming out into a less dense material (glass → air) light speeds up and bends away from the normal.

CCEA note: the bigger the change of speed, the bigger the refraction. You relate the amount of bending to the change of speed — a formula for Snell's law is not required.

Which way does it bend?

A prism splits white light

White light is a mixture of colours. A glass prism spreads it into a spectrum — this is dispersion. It happens because the different colours travel at slightly different speeds in the glass, so they are refracted by different amounts.

Order of the spectrum: red, orange, yellow, green, blue, violet. Violet bends the most, red the least.

Most or least refracted?

Tap the colour that is refracted the way each clue describes.

The critical angle & total internal reflection

When light travels from a denser to a less dense material (glass → air), increasing the angle of incidence inside the glass refracts the ray further from the normal. There is a special angle — the critical angle — where the refracted ray runs along the surface (angle of refraction = 90°).

• At the critical angle: the angle of refraction in the air becomes 90°.
• Above the critical angle: no light escapes — it is all reflected back inside. This is total internal reflection (TIR).

Optical fibres & prisms

An optical fibre is a thin glass strand. Light shone in at one end hits the wall at an angle greater than the critical angle, so it is totally internally reflected again and again — bouncing all the way along, even round bends.

TIR is also used in prisms — e.g. a 45° glass prism turns light through 90° or 180° in binoculars and periscopes, because the inside angle exceeds the critical angle.

When does TIR happen?

Converging & diverging lenses

A lens refracts light to form an image. CCEA needs you to tell the two types apart:

• A converging (convex) lens is fatter in the middle. Parallel rays are brought together at the principal focus (F).
• A diverging (concave) lens is thinner in the middle. Parallel rays are spread apart, appearing to come from a focus behind the lens.

CCEA note: lenses are treated qualitatively. You should be able to define the focal length of a converging lens and tell the two lens types apart — no lens-power or magnification formula is required.

Which lens does which job?

Tap a use on the left, then tap the lens type that does it.

A converging lens forming a real image

When the object is beyond the principal focus F, a converging lens forms a real image — one that light actually reaches, so it can be caught on a screen. Two key rays fix where it forms:

• A ray parallel to the axis refracts through F on the far side.
• A ray through the centre of the lens carries straight on.

Your turn — focal length

A converging lens as a magnifying glass

Move the object closer than the principal focus F and the refracted rays diverge — they never meet on the far side. Trace them backwards and they appear to come from a large, upright, virtual image on the same side as the object. That is a magnifying glass.

Watch out: the same converging lens gives a real image when the object is beyond F, but a virtual image when it is inside F. The image type depends on the object distance.

Real or virtual?

Short sight & long sight

The eye uses a converging lens to focus light onto the retina. Sometimes it doesn't focus there — and a corrective lens fixes it:

• Short sight (myopia): the eyeball is slightly too long, so distant objects focus in front of the retina and look blurred. Corrected with a diverging lens.
• Long sight (hyperopia): the eyeball is too short (or the lens too weak), so near objects would focus behind the retina. Corrected with a converging lens.

Correcting vision

The Light essentials

You've covered all of CCEA 2.2 — Light: reflection, refraction, dispersion, total internal reflection, optical fibres and lenses. Press Finish to see your score.