FORM 4: 5.6 FORMULA

THE LAWS OF REFLECTION

REFRACTIVE INDEX, n

POWER OF A LENS

MAGNIFICATION, m

LENS FORMULA

FORM 4: 5.5 MEANINGS

FORM 4: 5.4 UNDERSTANDING LENSES

CONVEX LENSES

A convex lens is thicker at its centre than at the edges.

CONCAVE LENSES

A concave lens is thinner at its centre than at the edges.

FOCAL POINT AND FOCAL LENGTH OF A LENS

1. Focal point, F is the point where the rays converge to it after passing through the lens.
2. Focal length, f is the distance between the focal point and the centre of the lens.

THE MEANING OF POWER OF LENSES

The power of a lens is defined as the ability to bend a light ray entering it.

• The higher the power of a lens, the shorter its focal length and vice versa.
• The thicker the centre of a convex lens, the shorter the focal length of the lens, the higher the power of the lens.

RAY DIAGRAM OF THE IMAGE FORMED BY A CONVEX LENS

RAY DIAGRAM OF THE IMAGE FORMED BY A CONCAVE LENS

MAGNIFICATION, m

LENS FORMULA

THE USE OF LENSES IN OPTICAL DEVICES

FORM 4: 5.3 UNDERSTANDING TOTAL INTERNAL REFLECTION

• When light enters from a denser medium (glass) to less dense medium (air), it is bent or refracted away from the normal.
•  A small part of the incident ray is reflected inside the glass.

•  As the angle of incidence, i increases, the angle of refraction, r also increases.
• When the angle of refraction reaches 90^o, the light is refracted along the boundary.
• This angle of incidence in the denser medium is called the critical angle, c.
• The critical angle is defined as the angle of incidence in the denser medium when the angle of refraction in the less dense medium is 90^o.

•  If the angle of incidence is increased further so that it is greater than the critical angle, the light is not refracted anymore, but is internally reflected.
• This phenomenon is called total internal reflection.

THE MEANING OF TOTAL INTERNAL REFLECTION

Total internal reflection occurs when a light ray passes from a denser medium towards a less dense medium with the angle of incidence greater than the critical angle of the medium.

THE CONDITIONS FOR TOTAL INTERNAL REFLECTION TO OCCUR ARE:

1. light ray enters from a denser medium towards a less dense medium
2. the angle of incidence in the denser medium is greater than the critical angle of the medium.

REFRACTIVE INDEX, n

NATURAL PHENOMENA INVOLVING TOTAL INTERNAL REFLECTION 

• The layers of air near the ground are hotter and less dense. The upper layers are cooler and denser.
• When light rays from the sky travel towards the Earth’s surface, the light ray are refracted away from the normal due to decreasing air density.
•  The angle of incidence increases as the light rays enter the next layer of air.
• The light rays are reflected when the angle of incidence exceeds the critical angle of air in that layer.

APPLICATIONS OF TOTAL INTERNAL REFLECTION

• An optical fibre consists of an inner core of high refractive index glass and surrounded by an outer cladding of lower refractive index.
• When light is introduced into the inner core at one end, it will undergo a series of total internal reflection.

FORM 4: 5.2 UNDERSTANDING REFRACTION OF LIGHT

THE MEANING OF REFRACTION OF LIGHT

Refraction of light is the phenomenon where light change direction at the boundary between two materials of different optical densities.

Light didn’t bend when travelling along normal.

Light bends toward the normal when it enters an optically denser medium.

Light bends away from the normal when it enters an optically less dense medium.

REFRACTIVE INDEX, n

THE LAWS OF REFRACTION / SNELL’S LAW

NATURAL PHENOMENA DUE TO REFRACTION

FORM 4: 5.1 UNDERSTANDING REFLECTION OF LIGHT

PLANE MIRROR

We are able to see because light from an object can move through space and reach our eyes.

THE LAWS OF REFLECTION

1.       The incident ray, the reflected ray and the normal all lie in the same plane.

2.       The angle of incidence, i is equal to the angle of reflection, r.

RAY DIAGRAM OF THE IMAGE FORMED BY A PLANE MIRROR

THE CHARACTERISTICS OF THE IMAGE FORMED BY A PLANE MIRROR:

1.       laterally inverted

2.       same size as the object

3.       virtual

4.       as far behind the mirror as the object as the object is in front of it

CURVED MIRROR

1.       The centre of curvature, C of curved mirror is the centre of sphere of the mirror.

2.       The radius of curvature, R is the distance between the centre of curvature, C and the surface of the mirror.

3.       The principal axis is the connecting line from the centre of curvature to point P.

4.       The focal point, F is the point on principal axis where parallel rays that strike the surface of a convex mirror will be reflected and the diverging rays appear to come from a point inside the concave mirror.

5.       The focal length, f is the distance between the centre of curvature, C and the focal point, F. (CF = FP)

RAY DIAGRAM OF THE IMAGE FORMED BY A CONCAVE MIRROR

RAY DIAGRAM OF THE IMAGE FORMED BY A CONVEX MIRROR

APPLICATIONS OF PLANE MIRROR

MIRROR IN A METER

The correct reading can only be taken when the image of pointer is exactly under the pointer.

SIMPLE PERISCOPE

§   A simple periscope consists of two plane mirrors inclined at an angle of 45^o.

§   A periscope can be used to see over the top of high obstacles such as a wall.

§   It is also used inside a submarine to observe the surrounding above water surface.

OHP (OVERHEAD PROJECTOR)

OHP uses plane mirror to reflect the image to the screen.

APPLICATIONS OF CONCAVE MIRROR

TORCHES & CAR HEADLAMP

A small lamp is placed at the focus point of the mirror to produce parallel rays.

APPLICATIONS OF CONVEX MIRROR

SECURITY MIRROR

A security mirror made from a convex mirror helps to widen field of vision for security purposes.