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.

Refraction of light by a thin convex lens. 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

ff

f f

f

Optic centre

Formation of an image by a convex lens. For a convex lens;

1. If the object is outside the focus, the image is real, located at the opposite side of the lens and is inverted.

2. If the object is inside the focus, the image is virtual, located at the same side of the lens and is upright.

Object Lens

Image

N.B. A real image of a distant object forms at the focus of a convex lens.

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.

1 1 1 f u v

Magnification m = v u

Exercise 5.1 pg. 48!

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

ff

f

f f

Formation of an image by a concave lens.

For a concave lens;

1. The image is always virtual, located at the same side of the lens as the object, and upright.

2. The image is always diminished but increases as the object approaches the lens.

f

f

f

f

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 lengthP = 1 f

f

f

Shorter focal length = greater power.

Longer focal length = less power.

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

                            

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.

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.

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.

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.