Images in Mirrors and Lenses

What is an image?

Light goes in straight lines. When light from an object hits a mirror it bounces. When it goes through a lens it bends. Our eye follows the rays back in a straight line and sees a copy of the object. This copy is the image.

A real image forms where the rays truly meet. You can catch it on a paper screen (like a cinema picture). A virtual image forms where the rays only seem to come from. It is behind the mirror, so no screen can catch it.

Image in a plane mirror

A plane mirror is a flat mirror. Its image is:

• virtual (behind the mirror),
• upright (the same way up),
• the same size as the object,
• as far behind the mirror as the object is in front,
• left and right swapped (this is called lateral inversion). That is why the word AMBULANCE is written backwards on the front of the van.

If you stand 2 m from a plane mirror, your image is 2 m behind it. You and your image are 4 m apart.

Concave and convex mirrors

A concave mirror curves inward like the inside of a spoon. It brings light together, so it is a converging mirror. The point where parallel rays meet is its focus (F).

• Object far away (beyond F): the image is real, upside-down, and in front of the mirror. Far objects give small images.
• Object very close (inside F): the image is virtual, upright and bigger. Shaving mirrors, make-up mirrors and the dentist's mirror use this.
• Object exactly at F: the rays leave parallel and no image forms. A torch or car headlight uses this to make a straight beam.

A convex mirror bulges outward. It spreads light out. Its image is always virtual, upright and smaller, for every object position. Small images mean a wide view, so it is used as a side mirror and in shop security mirrors.

Images in lenses

A convex lens is thick in the middle and brings rays together. It acts like a concave mirror: far object gives a real, upside-down image on the other side; a close object (inside F) gives a big, upright, virtual image. That is a magnifying glass. A camera, a projector and your eye use the real-image case.

A concave lens is thin in the middle and spreads rays out. Like a convex mirror, it always gives a small, upright, virtual image. Glasses for short sight use it.

In the 3D picture, the orange and green lines are two easy rays. Dashed blue lines show where a virtual image only seems to be.

Try it

At home: Look at your face in the inside and then the outside of a shiny steel spoon. The inside (concave) shows you upside-down and small. The outside (convex) shows you upright and small. Now bring the inside of the spoon very close to your eye: your face gets big and upright.

In the 3D: first guess if the image will be real or virtual, then slide the object and check.

Key formulas and definitions

• Plane mirror: image distance = object distance (virtual, upright, same size)
• Real image: can be caught on a screen, usually upside-down
• Virtual image: cannot be caught on a screen, upright in these cases
• Mirror and lens formula (real distances positive): 1/v + 1/u = 1/f
• Magnification m = image height / object height = − v / u

Worked examples

Common mistakes

• Thinking a virtual image is not there. You can see it; you just cannot catch it on a screen.
• Saying a plane mirror makes the image smaller. It is the same size.
• Mixing up the two mirrors: concave curves inward (cave = hollow), convex bulges outward.
• Forgetting that a close object (inside F) gives a different image than a far one for concave mirrors and convex lenses.