Physics
Mirrors & lenses
From the bathroom mirror to your phone camera: how we steer, focus, and use light.
What you need first
In the morning you check yourself in the mirror, during the day you take photos with your phone camera, and maybe you wear glasses while doing it. All three things do the same job: they steer light in a very particular way. Once you know the two basic tricks, you understand mirrors, magnifiers, cameras, and even your own eye.
Mirrors: light bounces back in an orderly way
A rough wall scatters light in all directions, which is why you see no image in it. A mirror is so smooth that every light ray bounces back in an orderly way. A simple rule applies: the angle of incidence equals the angle of reflection. Both angles are measured from the normal, the imaginary line standing perpendicular on the mirror. If a ray arrives at 30 degrees, it leaves the mirror at 30 degrees too, just on the other side of the normal. The angle between the incoming and the reflected ray is made up of these two angles, here 30 degrees plus 30 degrees, which is 60 degrees. So it is always twice the angle of incidence.
Light travels in straight lines.
Lenses: focusing or spreading light
A lens is a transparent, curved piece of glass or plastic that redirects light rays. Converging lenses are thick in the middle and focus parallel light into one point, the focal point. The distance from the lens to that point is the focal length. That is how a magnifying glass works: hold it closer to the text than its focal length and you see the letters enlarged. Diverging lenses are thin in the middle and spread light apart. A camera uses converging lenses to project a sharp, small image of the world onto its sensor.
Your eye: a camera made of flesh and blood
Your eye is itself a lens system: the cornea and the eye's lens focus light so that a sharp image forms on the retina. If the focusing and the length of the eyeball do not match, the image lands in front of or behind the retina and turns blurred: you are short-sighted or long-sighted. Glasses simply place a suitable lens in front that corrects the error: diverging lenses help the short-sighted, converging lenses the long-sighted. That's exactly why every glasses prescription lists a strength in dioptres.
Exercises
0 of 6 solvedTime to try it yourself. You can't break anything, every attempt counts.
Which rule applies at a flat mirror?
A light ray hits a mirror at an angle of incidence of 30 degrees. What is the angle of reflection in degrees?
A ray hits the mirror exactly along the normal, so the angle of incidence is 0 degrees. At what angle in degrees is it reflected?
At a flat mirror the angle of incidence is exactly as large as the ….
Which lens is inside a magnifying glass?
Match each part to what it does with light.
Where this leads