Lesson Topics

1 | Refraction

2 | The Refractive Index and Law of Refraction

3 | Lenses and Refraction of Light

4 | Lenses and Image Formation

5 | Lenses in the Eye

6 | Symmetrical Lenses

7 | Drawing Ray Diagrams for Convex Lenses

8 | Drawing Ray Diagrams for Concave Lenses

9 | Real and Virtual Images from Lenses

10 | Summary

Lesson Summary

• Refraction is a process by which light waves are bent due to changing speed when moving from one medium to another.

• The refractive index of a substance is a measure of light’s relative speed in that substance and therefore a measure of how far light will bend when entering that substance from air.

• The law of refraction (Snell’s law) states that when light enters a substance with a higher refractive index, it bends towards the normal, and when light enters a substance with a lower refractive index, it bends away from the normal.

• A lens is a curved, transparent piece of glass or plastic that refracts light.

• A convex lens curves outwards and is thicker at the centre.

• A concave lens curves inwards and is thinner at the centre.

• When light rays pass through a convex lens they converge.

• When light rays pass through a concave lens they diverge.

• Convex lenses produce images that vary depending on the distance between the object and the lens.

• Concave lenses always produce images that are upright and diminished.

• Human eyes contain a convex lens that focuses incoming light onto the retina at the back of the eye.

• Near-sightedness is a condition where people can’t see distant objects clearly.

• It can be corrected with concave lenses.

• Far-sightedness is a condition where people can’t see near objects clearly.

• It can be corrected with convex lenses.

• Symmetrical lenses have a focal point on each side of the lens, with equal focal lengths.

• The mid-plane of a symmetrical lens divides the lens in half.

• The optical centre of a symmetrical lens is the geometrical centre, where no refraction occurs.

• The principal axis is a straight line passing through the optical centre of a lens, perpendicular to the mid-plane.

• A gently curved lens causes less refraction and therefore has a longer focal length.

• A strongly curved lens causes greater refraction and therefore has a shorter focal length.

• Ray diagrams can be used to predict the size and orientation of images produced by lenses.

• Rules for drawing ray diagrams for convex lenses:

• Light rays that are parallel to the principal axis will pass through the focal point after refraction.

• Light rays that pass through the optical centre will continue in a straight line without refraction.

• Light rays that pass through the focal point before the lens will become parallel to the principal axis after refraction.

• Rules for Drawing Ray Diagrams for Concave Lenses:

• Light rays that are parallel to the principal axis will appear to come from the opposite focal point after refracting through the lens.

• Light rays that pass through the optical centre will continue in a straight line without refraction.

• Light rays that are directed at the opposite focal point will become parallel to the principal axis after refraction.

• A real image is an inverted image formed on the opposite side of a lens as an object, when light rays from the object converge after refraction.

• A virtual image is an upright image formed on the same side of a lens as an object, when light rays from the object diverge after refraction.

(Image: Kranich17, Pixabay)

(Header image: korionov, Adobe Stock)