Refraction of Light

Refraction of light occurs when light travels from one material to another.

normal

normal

ir

ir

AIR

GLASS

i – angle of incidence

r – angle of refraction

AIR into GLASS

The light bends towards the normal.

GLASS into AIR

The light bends away from the normal.

The angle of incidence and refraction is always measured in relation to the normal.

Focus

A convex lens brings the rays of light together to a focus.

Lenses

Convex Lens

Concave Lens

A concave lens spreads the rays of light further apart.

Power of a Convex Lens

Thin Convex Lens

f

The distance from the lens to the focus is called the

focal length.

A thin convex lens has a long focal length.

Thick Convex Lens

A thick convex lens has a short focal length.

A thicker convex lens brings the rays of light to a focus in a shorter distance.

A thicker convex lens is more powerful.

f

Equipment needed to find the focal length of a convex lens:

• Convex lens

• Sheet of paper

• Ruler

• Distant object

Focal Length of a Convex Lens

Using the apparatus you should:

• Arrange the lens until an image of the distant object can be seen on the paper.

• Move the lens until the sharpest image can be seen.

• Measure the distance between the lens and the paper with the ruler.

x

÷

Convex Lens

The power and focal length of a lens are linked by an equation:

Power and Focal Length

1

P f

Quantity Unit

Power ( P )

Focal Length ( f )

Dioptres (D)

Metres ( m )

Example

A convex lens has a power of 5 D. Calculate the focal length of such a lens.

D 5P???f

P1

f

51

m 0.2f

Example

A convex lens has a focal length of 25 cm. What is the power of the lens?

???P

cm 25f f1

P

0.251

D 4P

m 0.25

Convex lens have POSITIVE Powers and Focal Lengths.

1. Calculate the focal lengths of these convex lens if their powers are:

(a) 10 D

(b) 20 D

(c) 2 D

(d) 12.5 D

2. Calculate the powers of these convex lens if their focal lengths are:

(a) 5 cm

(b) 40 cm

(c) 50 cm

(d) 16 cm

Attempt the following questions showing ALL your working.

0.1 m

0.05 m

0.5 m

0.08 m

20 D

2.5 D

2 D

6.25 D

A concave lens does not bring the rays of light to a focus.

However, the rays can be extended backwards to make a virtual focus.

The focal length is in the opposite direction so it is a NEGATIVE number.

f

Power of a Concave Lens

Example

A concave lens has a focal length of – 20cm. Calculate the power of this lens.

???P

cm 20f

f1

P

0.2-1

D 5P m 0.2

Example

A concave lens has a power of -2.5 D. What is the focal length of the lens?

D 2.5P ???f

P1

f

2.5-1

m 0.4f

Concave lens have NEGATIVE Powers and Focal Lengths.

1. Calculate the focal lengths of these concave lens if their powers are:

(a) -1 D

(b) -2 D

(c) -4 D

(d) -2.5 D

2. Calculate the powers of these convex lens if their focal lengths are:

(a) -50 cm

(b) -40 cm

(c) -25 cm

(d) -60 cm

Attempt the following questions showing ALL your working.

-1 m

-0.5 m

-0.25 m

-0.4 m

-2 D

-2.5 D

-4 D

-1.67 D

The Eye

Light enters our eyes via the pupil.

Optic Nerve

Retina

Lens

Iris

Pupil

Cornea

Pupil

Eyelid

Iris

Eyelashes

White of The Eye

(sclera)

Iris

Pupil

Pupil size in dim light

Pupil size in bright

light

The light is focussed on the retina by the cornea and the lens.

The image formed on the retina is:

• upside down

• back-to-front (laterally inverted)

object

image

focus

Normal Sight

A person with normal sight can see near and distant objects.

This is done by changing the thickness of the convex lens inside the eye.

Distant Objects

Parallel rays from a point on a

distant object.

Muscles stretch the convex lens inside the eye to make it just thin enough to bring the rays of light from a point on a distant object to a focus on the retina.

Near Objects

Diverging rays from a point on a

near object.

Muscles squash the convex lens inside the eye to make it just thick enough to bring the rays of light from a point on a near object to a focus on the retina.

Long Sight

A long sighted person can focus clearly on distant objects.

Near objects appear blurred.

Diverging rays from a point on a

near object.

The muscles can’t make the convex lens thick enough.

Not enough refraction takes place.

When the rays strike the retina, they’re apart.

Long sight can be corrected by wearing glasses or contacts with convex lens.

Diverging rays from a point on a

near object.

The convex lens in front of the eye starts to bring the rays closer together.

The convex lens inside the eye can now bring the rays of light to a focus on the retina.

Short Sight

A short sighted person can focus clearly on near objects.

Distant objects appear blurred.

Parallel rays from a point on a

distant object.

The muscles can’t make the convex lens thin enough.

Too much refraction takes place, the focus is in front of the retina.

When the rays strike the retina, they’re apart so object’s blurred.

Short sight can be corrected by wearing glasses or contacts with concave lens.

The concave lens in front of the eye spreads the rays apart.

The convex lens inside the eye has more refraction to do.

This pushes the focus further back.

If the correct power of concave lens is used then the rays will come to a focus on the retina.

Parallel rays from a point on a

distant object.

The Endoscope

An endoscope allows doctors to see inside a patients stomach or lungs.

It consists of two bundles of optical fibres (one coherent fibre and one incoherent fibre).

A coherent fibre is needed to get a decent image.

An incoherent fibre can be used to transmit light to the tip of an endoscope.

Only light is able to pass through an optical fibre. This means the heat from the light source is not transmitted into the body.

This is called a ‘cold light’ source.

coherent fibre

incoherent fibre

(jumbled up)

incoherent bundle

protective sheath

coherent bundle

light source

eyepiece lens

focussing ring

light guide exit

lens

How it Works…

The endoscope is placed inside the body.

The first optical fibre sends ‘cold light’ into the body.

The second optical fibre carries reflected light back so an image of inside the body can be seen.