Reflection, refraction and lenses

Refraction through a rectangular glass block is the standard experiment and the standard ray diagram, and both are worth being able to produce from memory.

Place the block on paper, draw round it, and shine a narrow ray from a ray box at the long face. Mark the incoming ray with two crosses, and the emerging ray with two more, then remove the block and join them up through it. Draw the normal — a dashed line at 90° to the surface — at the point where the ray enters, and measure the angles from the ray to the normal, never to the surface.

Entering the glass, the ray slows and bends towards the normal. Leaving it, the ray speeds up and bends away again. Because the two faces are parallel, the emerging ray ends up parallel to the original one but shifted sideways.

Total internal reflection happens when light tries to leave a dense material for a less dense one — glass to air, or water to air — and two conditions are both needed:

• the light must be travelling from the more dense material into the less dense one • the angle of incidence must be greater than the critical angle for that boundary.

Below the critical angle, most of the light refracts out and a little reflects back. Exactly at the critical angle, the refracted ray travels along the surface. Above it, none escapes: all of the light is reflected back inside, obeying the ordinary law of reflection.

The uses follow directly. An optical fibre keeps light bouncing along its core because every strike on the wall is above the critical angle, which is how a signal travels kilometres without leaking out. A prism periscope and the reflectors in a bicycle light work the same way.

Each colour within the visible light spectrum has its own narrow band of wavelength and frequency.

Reflection from a smooth surface in a single direction is called specular reflection. Reflection from a rough surface causes scattering: this is called diffuse reflection.

Colour filters work by absorbing certain wavelengths, and colour, and transmitting other wavelengths, and colour.

The colour of an opaque object is determined by which wavelengths of light are more strongly reflected. Wavelengths that are not reflected are absorbed. If all wavelengths are reflected equally the object appears white. If all wavelengths are absorbed the object appears black.

Objects that transmit light are either transparent or translucent.

You should be able to explain how the colour of an object is related to the differential absorption, transmission and reflection of different wavelengths of light by the object; the effect of viewing objects through filters, or the effect on light of passing through filters; and why an opaque object has a particular colour.

The filter questions follow directly. A red object viewed through a red filter still looks red, because the red light it reflects is transmitted by the filter. The same red object viewed through a blue filter looks black — it reflects only red light, and the blue filter absorbs red, so no light reaches the eye.

A lens forms an image by refracting light. In a convex lens, parallel rays of light are brought to a focus at the principal focus. The distance from the lens to the principal focus is called the focal length.

Ray diagrams are used to show the formation of images by convex and concave lenses. The image produced by a convex lens can be either real or virtual. The image produced by a concave lens is always virtual.

You should be able to construct ray diagrams to illustrate the similarities and differences between convex and concave lenses.

For a ray diagram, draw two rays from the top of the object:

• one travelling parallel to the axis, which is refracted through the principal focus • one passing straight through the centre of the lens without bending.

Where they meet is the top of the image. If they actually cross, the image is real and is drawn inverted; if they only appear to come from a point when traced backwards, the image is virtual and is drawn upright with dashed rays.

In ray diagrams a convex lens is represented by a vertical line with outward-pointing arrowheads at each end, and a concave lens by one with inward-pointing arrowheads.

The magnification produced by a lens can be calculated using the equation:

magnification = image height / object height

This equation is given on the Physics equation sheet. Magnification is a ratio and so has no units. Image height and object height should both be measured in either mm or cm — the same unit for both, so that the units cancel.