Form 4 Physics Next > The study of matter Chapter 5: Light 1

Physics: Chapter 5 Objectives: (what you will learn) 1) understanding Light 2) understanding the Laws < Back Next > 2

Light Reflection 3 Light travels in straight lines. When light is incident on a surface, it is reflected. Laws of reflection of light a. The incident ray, the reflected ray and the normal are in the same plane. b. The angle of incidence is equal to the angle of reflection. < Back Next > Characteristics of images formed by plane mirror a. Same size as object b. Virtual c. Laterally inverted d. Upright. e. The image is as far behind the mirror as the object is in front 3

Refraction 4 Laws of refraction When light travels from one medium to another medium of different optical density, its speed changes. This causes the change in its direction and the light bends. The bending of light is called refraction. < Back Next > Laws of refraction a. The incident ray, the refracted ray and the normal lie on the same plane. b. The ratio of sine of angle of incidence to sine of angle of refraction is a constant (Snell’s Law). When light travels from an optically less dense to a denser medium, it bends towards the normal. When light travels from an optically denser to a less dense medium, it bends away from the normal. 4

Refraction Refractive index of medium X may be found by the following: n = Sine (angle of incidence) Sine (angle of refraction) < Back Next > n = Velocity of light in air Velocity of light in medium X n = Actual depth Apparent depth http://en.wikipedia.org/wiki/Mirage n = 1 Sine C , C = critical angle 5

Mirage Mirage A naturally-occurring optical phenomenon, in which light rays are bent to produce a displaced image of distant objects or the sky. The interpretation of the image is up to the fantasy of human mind, and is usually mistaken for a small puddle of water. The word comes to English via the French ‘mirage’, and from the Latin ‘mirare’, meaning 'to appear, to seem'. It has the same root as for mirror. Like a mirror, a mirage shows images of things which are elsewhere. The principal physical cause of a mirage, however, is refraction rather than reflection. It is also caused by the phenomenon of total internal reflection which is the result of refraction. < Back Next > http://en.wikipedia.org/wiki/Mirage 6

Mirage 7 Inferior Mirage Superior Mirage Cold air is denser than warm air, and has a greater refractive index. As light passes from colder air above to warmer air below it bends away from the normal, resulting in an inferior image for the sky above. < Back Next > http://en.wikipedia.org/wiki/Mirage Superior Mirage Where the air near the ground is cooler than that higher up, the light rays will curve downwards, producing a superior image. Superior mirages are most common in polar regions, especially over large sheets of ice with a uniform low temperature. 7

Internal Reflection Critical angle is angle of incidence when light travels from denser medium to less dense medium and angle of refraction is 90o. C Normal Less dense medium Denser medium < Back Next > When light travels from denser to less dense medium and the critical angle is exceeded, total internal reflection takes place. Application of total internal reflection: optical fibres A light ray enters fibre at an angle > critical angle Total internal reflection takes place along whole length of the fibre. 8

Lens 9 There are 2 types of lenses: convex and concave lenses Convex lens: the point where refracted rays meet is called focal point Concave lens: the light rays seem to be diverged from a single point, the point is the focal point Focal length (f): distance from optical centre to focal point < Back Next > Power of lens = 1 f (in metre) , unit is dioptre (D) A real image can be formed on a screen. A virtual image cannot be formed on a screen. Optical instruments which use lenses: Camera Photocopier Magnifying glass Slide projector Microscope Telescope 9

Lens 10 1 1 1 u v f + = Lens equation: v u Magnification of lens = Object distance (u) Image distance (v) Characteristics of image Uses u < f Same side as object Virtual, upright, enlarged Magnifying glass u = f At infinity Telescope f < u < 2f v > 2f Real, inverted, enlarged Slide projector u = 2f v = 2f Real, inverted, same size Photocopier u > 2f F < v < 2f Real, inverted, diminished camera < Back Next > 1 1 1 u v f + = Lens equation: 10 v u Magnification of lens =

Summary 11 What you have learned: Thank You Understanding Light < Back Understanding Light 2. Reflection and Refraction 3. Using the Laws 11 Thank You