Concave Mirror
Slide the object towards the mirror, past C and then past F, and follow its image.
Upside down and smaller, in front of the mirror, where a screen could catch it. Like a big telescope mirror looking at the Moon.
- Object distance
- 30 cm
- Image distance
- 15 cm in front
- Size
- 0.5 × upside down
What’s happening
Look into the bowl of a shiny spoon: you see yourself small and upside down. Bring it close to your eye and you flip the right way up and grow. A concave mirror curves inwards like that spoon, and it sends light inwards too, so the light from the object’s tip meets again in front of the mirror: an upside-down picture you could catch on a screen. Inside the focus F the light can’t meet any more. It spreads out, and seems to come from a bigger, upright image behind the mirror, the one you see in a make-up mirror.
Try this
- Find a spot where the image is upside down and bigger than the object.
- Put the object on C, the centre of the mirror’s curve. Where is the image, and how big?
- Put the object exactly on F. Where do the reflected rays go?
- Cross F. Where does the image jump to?
- At home: look into the bowl of a spoon, first at arm’s length, then very close. When do you flip?
Learn more: the three rays, the mirror equation, and dishes that gather sunlight and radio waves
A mirror has three rays of its own. Any two of them find the image:
- Parallel ray: comes in parallel to the axis, and reflects through the focus.
- Middle ray: hits the middle of the mirror, and bounces off at the same angle, like a ball off a wall.
- Focal ray: comes in through the focus, and reflects parallel to the axis.
The mirror equation is the same as the lens equation, where is the object distance, the image distance and the focal length:
The focus is halfway between the mirror and C, the centre of the ball the mirror’s curve would make: the focal length is half that ball’s radius .
- Beyond C: smaller and upside down (a telescope mirror).
- At C: the same size, upside down, right under the object.
- Between F and C: bigger and upside down.
- At F: no image; the light leaves in a parallel beam (a torch or a headlight, with the bulb at F).
- Inside F: bigger and upright, behind the mirror (a make-up or shaving mirror).
Satellite dishes are concave mirrors for radio waves: they gather a weak signal from space onto the receiver at their focus. Solar cookers do the same with sunlight, to heat a pot.
The drawing uses an ideal mirror, where every ray obeys the equation exactly. A real mirror only does that for rays close to the axis: rays that hit it far from the middle meet a little closer to it, and the image blurs. Telescope mirrors are made a slightly different shape, a parabola, so that light from far away all meets at one point.