Lenses

1. Lenses • Lenses display focusing properties because of refraction. • A convex lens will focus a parallel beam of light to a certain point. • A concave lens will diverge a parallel beam of light and it appears to have come from a particular point.

3. Refraction of light by a thin convex lens. Optic centre • A ray which strikes the optic centre passes straight through the lens. • A ray travelling parallel to the axis is refracted through the focus at the opposite side of the lens. • A ray passing through the focus and striking the lens is refracted parallel to the axis. f f f f f f

4. Formation of an image by a convex lens. • For a convex lens; • If the object is outside the focus, the image is real, located at the opposite side of the lens and is inverted. • If the object is inside the focus, the image is virtual, located at the same side of the lens and is upright. Lens Object Image N.B. A real image of a distant object forms at the focus of a convex lens.

5. The Lens Formula. • The object distance ‘u’ is always positive. • The focal length ‘f’ is positive for a convex lens and negative for a concave lens. • A negative value for ‘v’ indicates a virtual image, a positive value for ‘v’ indicates a real image. 111 f u v Magnification m = v u Exercise 5.1 pg. 48!

6. Refraction of light by a thin concave lens. • A ray which strikes the optic centre passes straight through the lens. • A ray travelling parallel to the axis is refracted as if it came from the focus. • A ray coming from the focus is refracted parallel to the axis. f f f f f f

7. Formation of an image by a concave lens. • For a concave lens; • The image is always virtual, located at the same side of the lens as the object, and upright. • The image is always diminished but increases as the object approaches the lens. f f f f

8. Power of a lens. • The shorter the focal length ‘f ’ of a lens, the quicker it can focus or diverge a parallel beam of light. • The power of a lens is defined as; Power = 1 / focal length P = 1 f f Shorter focal length = greater power. f Longer focal length = less power.

9. Power of a combination! • If two lenses of power P1 and P2, are placed in contact, the power P of the combination is given by, P = P1 + P2 • It follows that the focal length ‘f ’of a combination of lenses can be given by, 1 = 1 + 1 f f1 f2 N.B.f is + for a convex lens f is – for a concave lens

10. The Human Eye. • When light from an object enters the eye, a real inverted image is formed on the retina. • The brain detects this image as upright. • The cornea, lens, aqueous humour and vitreous humour form the focusing system of the human eye.

11. Power of Accommodation. • The iris controls the amount of light entering the eye through the pupil. • The ciliary muscles attached to the lens can relax or contract to change the shape of the lens of the eye. • This allows the eye to focus on near or far objects in quick succession. • When the ciliary muscles are relaxed the lens is at its thinnest and will focus a distant object. • When contracted the lens is fattened (shorter focal length), and can focus a near object.

12. Vision Defects • A short sighted person (myopia) cannot properly focus the image of a distant object onto the retina. • The image appears blurred. • Myopia can be corrected with a concave lens. Myopia: Parallel light from a distant object is focused short of the retina and appears blurred. A concave lens can correct short sight.

13. Hyperopia or long sight is when a person cannot bring the image of a near object into focus on the retina of the eye. • Without the help of a corrective lens, the image of a near object is formed past the retina. • Long sight may be corrected using a convex lens. Hyperopia: Long sight occurs when the eye cannot focus a near object. Hyperopia may be corrected with a convex lens.