GCSE Physics Light and lenses revision

A convex (converging) lens bulges in the middle and bends parallel rays inwards so they meet at the principal focus; the focal length is the distance from the lens centre to that focus. A concave (diverging) lens spreads parallel rays outwards as if they had come from a focus on the near side. Ray diagrams use two reliable rays: one straight through the centre of the lens, and one parallel to the axis that passes through the principal focus after the lens. Where the rays (or their backwards extensions) cross is the image. An image you can catch on a screen — where actual rays cross — is real; one formed only by rays that *appear* to come from it (like a magnifying glass view) is virtual.

For a convex lens the image depends on object distance: far beyond twice the focal length the image is real, inverted and diminished (a camera); just beyond the focal length it is real, inverted and magnified (a projector); inside the focal length the image is virtual, upright and magnified (a magnifying glass). A concave lens always produces a virtual, upright, diminished image — the view through a door peephole.

Magnification = image height ÷ object height. Because it is a ratio of two lengths it has no unit: a 3.0 cm object forming a 12 cm image has magnification 12 ÷ 3.0 = 4.0. Work backwards just as easily: magnification 2.5 with a 4.0 cm object gives an image of 2.5 × 4.0 = 10 cm. Magnifications above 1 mean enlarged, below 1 diminished, exactly 1 same-size.

Angles are always measured from the normal — the line at right angles to the surface. A smooth surface reflects light in a single direction (specular reflection) so a clear image forms; a rough surface reflects the same law ray-by-ray, but the tiny slopes point in random directions so the light scatters (diffuse reflection) — that is why paper is visible from every angle but shows no reflection of your face. When light crosses into glass or water it slows and bends towards the normal; leaving, it speeds up and bends away. Different substances bend light by different amounts.

1. 1Direct the ray

   Direct a narrow ray from a ray box at a flat mirror or transparent block set on a protractor-style paper.

2. 2Mark and measure incidence

   Mark the incident ray and the normal; measure the angle of incidence with a protractor.

3. 3Mark and measure the second ray

   Mark the reflected (or refracted) ray and measure its angle from the normal on the far side.

4. 4Repeat across angles and surfaces

   Repeat for incidence angles such as 10°, 20°, 30°, 40°, 50° — and compare a shiny mirror (specular: reflected angle = incident angle every time) with rougher or frosted surfaces (scattered rays, no sharp beam).

5. 5Try refraction

   For refraction, trace the ray through glass or water blocks of different materials and measure the angle of refraction from the normal — the bending differs with the material.

6. 6Keep it fair

   Control the variables: same ray-box width and brightness, surfaces flat and clean, measurements repeated and averaged.

White light is a mixture of colours; a prism disperses it because each colour slows by a different amount in glass. An opaque object looks the colour of the light it reflects — a red rose reflects red and absorbs the rest, so in green light (no red to reflect) it looks black. A filter transmits only its own colour: look at a blue sky through a red filter and the sky goes black. White objects reflect all colours; black objects absorb all colours. That same absorption shows up in infrared: the required practical compares how surfaces emit and absorb infrared, using a Leslie cube — a metal cube with four different faces filled with hot water — and an infrared detector to compare each face at the same temperature.