UNDERSTANDING REFRACTION OF LIGHT

Refraction of Light

1. The figure shows how light ray passes through a glass block. From the diagram,

• The light ray bends towards the normal when it travels from air to glass from a less densemedium to a denser medium.
• The light ray bends away from the normal when it leaves the glass to air from a denser medium to a less dense medium.
• The angle of refraction , r is always smaller than the angle of incidence, i from less dense medium to a denser medium.
• The emergent ray keeps constant direction. 

2. Refraction of light is the bending of a light ray at the boundary from one medium to another.

3. Any substance that a light ray travels through is called a medium.

Direction of refraction

• when the light travels from a less dense medium to a denser medium, the ray is refracted (toward/away from) the normal at point of incidence.
• when the light travels from a more dense medium to a denser medium, the ray is refracted (toward/away from) the normal at point of incidence.  

Snell's law

• - Snell's law state that the ratio at sin i to a sin r is constant.

Phenomena Due to Refraction of Light

Shallow Swimming pools

• A swimming pool that  appears to be shallower than it really is.
• this is because of light ray from the bottom of the pool being refracted away from the normal as it enter the air.

• When the light ray reaches the eye , it appears to come higher  up.
•  So, the pool appears to be shallower.

Bent Object in Water 

• the figure shows the pencil appearing to be bent at the air-water boundary.
• the light ray from the end of the pencil is refracted at the water-air boundary and bends away from the normal.

• The image of the pencil under the water appears to be higher than it really is.
• Thus, the pencil appears bent at the surface.  

A Fish - Eye View

• A fish or a diver can see object above the water surface , but the objects on the shore would appears to be up in the air at different positions as shown in the diagram .
• Refer to refraction of light , a fish sees the image of an object at a position that differs from its true position.

The Twinkling Stars

• Light from the stars travels in straight lines through the vacuum of space.
• The twinkling of stars at night is due to the refraction of light as it passes through the layers pf atmosphere of different densities. 

UNDERSTANDING REFLECTION OF LIGHT

LAW OF REFLECTION

a.  LAW (1) : the incident ray , the normal and the reflected ray at the point of incidence . all lie on the same plane.

b.  LAW (2) : the angle of incidence , i , is equal to the angle of reflection , r .DIAGRAM

APPLICATIONS OF REFLECTION OF LIGHT

(I)PLANE MIRROR

1. We usually look at ourselves in a plane mirror to make ourselves presentable.

2. Plane mirrors are fixed in barber shops to enable customers to see whether they are having a satisfactory hair-cut. Many shops are fixed with plane mirrors which can make shops look wider, besides being decorative.

3. Plane mirrors have been used extensively in optical instruments such as periscopes, overhead projectors (OHP) and cameras. Electric meters such as the ammeter and the voltmeter have a plane mirror under the pointer to help users obtain accurate readings. The correct position for the eye to take a correct reading with no parallax error is where the eye cannot see the image of the pointer.

4. a plane mirror fixed at one and of a ,metal rod is a very useful instrument for a dentist. By placing the plane mirror at the back of the teeth, a dentist can look at teeth through their image in the mirror.

(II)CONCAVE MIRRORS

1.  Concave mirrors used as shaving mirrors or make-up mirrors enable a person standing close to the mirrors to see their own enlarged, upright and virtual image.

2.  Concave mirrors are usually used as light reflectors in torchlights, car headlights and various searchlights (spotlights) to produce parallel beams of light.

3.  In the reflecting telescope invented by Newton , a concave mirror is used to focus parallel rays of light from a distant star. The Hale Telescope which is the largest reflecting telescope at the Mount Palomar observatory in California has a concave mirror with a diameter of about 5 m. such a large mirror can collect enough light energy to enable very distant stars and nebulae to be seen or photographed.

(ii(III) CONVEX MIRRORS

1.  Car rearview mirrors making use of plane mirrors are limited to only providing a narrow field of view.

2.  Convex mirrors are preferred as car rearview mirrors because they can provide a wider field of view than plane mirrors.

 For the same reason, large convex mirrors are often placed at sharp road corners or dangerous bends to enable the motorist to see the oncoming traffic