Chapter 7LIGHT, COLOUR AND SIGHT.

7.1 Formation of Images.

Formation of images by plane mirrors. images formed by a plane mirror are caused by reflection of light from the

mirror surface. the characteristics of an image formed by a plane mirror are :

a) virtualb) inversed laterallyc) uprightd) the same size as the objecte) image distance and object distance from the plane mirror are

the same.

formation of image on a plane mirror

Formation of images by lenses. the image formed by a convex lens depends on the distance of the object

from the lens. therefore, the characteristics of the image formed by a convex lens are

a) virtual or realb) upright or invertedc) diminished or enlarged

object

image

observer

plane mirror

normal

Convex lens Concave lensType of lens

Biconvex lens Biconcave lens

Role Converges light rays that passes through it.

Diverges light rays that passes through it.

Characteristics of image

For a distant object, the image formed is real, inverted and smaller than the object.

For a near object, the image formed is virtual, upright and bigger than the object.

Image formed is virtual, upright, and diminished.

Ray diagrams. a ray diagram is a diagram which shows the path of light rays passing

through a lens.

object distance is the distance between object and optical centre. image distance is the distance between image and optical centre. for a convex lens, light rays that are parallel with the principle axis will

converge at the focal point behind the lens.

Focus point

Optical centre

for a concave lens, light rays that are parallel to the principal axis will diverge after passing through the lens causing the focal point to be placed in front of the lens.

Construction of ray diagrams. there are specific procedures to follow when constructing ray diagrams. when drawing ray diagrams, the following should be adhered to :

all rays from the object should be drawn with solid lines with the direction marked.

virtual rays should be drawn using broken lines. real images should be drawn with solid lines and virtual images

should be drawn using broken lines.

Focus point

Optical centre

Ray diagrams of the formation of images by convex lens at different object distances.

Object more than 2F

The image formed is : real inverted smaller than the size of the object

Object at 2F

The image formed is : real inverted same size as the object

Object between F and 2F

The image formed is : real inverted larger than the size of the object

Object at F

The image formed is : image is formed at infinity

Object less than F

The image formed is : virtual upright larger than the size of the object

7.2 Formation of Image by Optical Instruments.

Optical Instruments.

Periscopes are normally used in submarines to look at objects on the water

surface. a periscope consists of two plane mirrors arranged parallel to one

another at an angle of 45o . the image formed by a periscope is

o virtualo uprighto the same size as the objecto at the same distance from the object.

the mirrors in the periscope can be replaced by a prism. the image formed will be clearer.

a periscope works based on the principle of the reflection of light

Magnifying glass a magnifying glass is used to observe small objects. the image formed is larger than the size of the object. is made of a biconvex lens. when holding a magnifying glass, the object distance is less than

the focal length thus, the image formed iso virtualo uprighto larger than the size of the object

a magnifying glass

a ray diagram showing the formation of image by a magnifying glass

Microscope is used to observe tiny object. consists of two convex lenses. when observing a tiny object, the tiny object must be out in front of

the objective lens. the objective lens forms an image which is

o realo invertedo larger than the size of the object

the eyepiece acts as a magnifying glass to enlarge the image formed by the objective lens iso virtualo invertedo enlarged

Telescope is used to look at distant objects. also consists of two concave lenses. the objective lens has a long focal length while the eyepiece has a

short focal length. a distant object is focused by the objective lens and forms an image

at the focal point iso realo invertedo smaller than the size of the object

the eyepiece acts as a magnifying glass to enlarge the real image formed by the objective lens iso virtualo invertedo larger than the size of the object

Camera a camera consists of three main parts – the lens, the shutter and

the film. the lens focuses the image of an object onto the film, the lens is

adjusted by the focus adjuster either towards or away from the film to obtain a sharp image of an object.

to focus on a nearby object, the lens is adjusted away from the film to obtain a sharp image.

to focus on a distant object, the lens is adjusted towards the film to obtain a sharp image.

light rays that enter will pass through the diaphragm. the diaphragm changes the size of the aperture to control the

amount of light entering the lens. the image on the film is real, inverted, and smaller than the size of

the object.

Part of camera FunctionLens (convex) Focuses the image of a distant object on the film.Aperture Allows light to enter the camera.Diaphragm To control the camera aperture that allows light to enter the

camera.Film Acts as a screen to receive an object image that captured on

it.

Comparison between eye and camera. the eye of human functions like a camera. the eye lens focuses light onto the retina to form an image.

comparison between eye and camera.

comparison of the structure of the eye and that of the camera to their functions.Part of the eye Function Part of the cameraEye lens Focuses light to form an image Camera lensIris Control the size of aperture and

amount of light that enterDiaphragm

Pupil Allows light to enter ApertureCiliary body Changes the size of lens to obtain

a sharp imageFocus adjuster

Retina Acts as a photosensitive layer to capture image

Film

the characteristics of an image formed on the retina of the eye are real, inverted, and smaller than the size of the object.

the ciliary body regulates the thickness of the eye lens. (this regulates the focal length of the eye lens)

when looking at a distant object, the ciliary body contracts and the lens becomes thinner.

when looking are a nearby object, the ciliary body relaxes and the lens becomes thicker.

the focus adjuster regulates the distance between the lens and the film to obtain a sharp image.

when taking a picture of a distant object, the focus adjuster is adjusted so that the lens is nearer to the film.

when taking a picture of a nearby object, the focus adjuster is adjusted so that the lens is further away from the film.

the size of the pupil and the amount of light entering the eye are controlled by the iris.

if we enter a dimly lit room, the size of the pupil enlarges and the amount of light entering the eye increases.

if we come out form a dark room to a brighter place and the amount of light entering the eye decreases.

the size of aperture and the amount of light entering the camera is controlled by the diaphragm.

when taking a picture under a dark conditions, the diaphragm increases the size of the aperture and the amount of light entering the camera increases.

when taking a picture under bright conditions, the diaphragm decreases the size of the aperture and the amount of light entering the camera decreases.

7.3 Light Dispersion.

is a process in which white light is split into its colour constituents called spectrum when it passes through a prism.

a spectrum consists of seven colours in this order :1. red2. orange3. yellow4. green5. blue6. indigo7. violet

light dispersion occurs because each colour constituent travels at different speed through the prism.

therefore, the coloured lights are refracted at different angles. violet light is refracted more by a glass prism compared to red light which

has a longer wavelength. this is because violet light travels at the lowest speed.

Formation of rainbow. is natural phenomenon related to the dispersion of light. a rainbow can be seen during a drizzle in the morning or afternoon when

sunlight shines on the raindrops. a raindrop acts as a prism. a rainbow is formed when sunlight passes through raindrops. the sunlight is retracted and dispersed into its colour constituents.

Formation of rainbow…

(a) sunlight shines on the raindrop.(b) a part of the sunlight is reflected off the outer surface of the raindrop.

(c) a part of the light is refracted at different angles in the raindrop.(d) the light is split into its colour constituents through dispersion.

(e) the colour constituents are reflected off the far inner surface of the raindrop.

(f) light refraction occurs again as it exits the raindrop.(g) a rainbow is formed in the sky.

7.4 Light Scattering. the earth’s atmosphere contains particles like gas molecules, vapour, and

dust. when white light incidents these particles, light rays are obstructed and

reflected. these light rays scatter in all directions. (this occurrence is called light

scattering) light scattering is related to light colour; blue light is scattered more

compared to red light because blue light has a shorter wavelength. lights with shorter wavelength are refracted more. examples of light scattering which occurs as natural phenomena are :

a) the blue sky during the dayb) the red sky during sunsetc) the red sun at sunset

blue light which a shorter wavelength is scattered more in all directions. this causes the sky to appear blue. at sunset, sunlight passes through a thicker atmospheric layer. red and orange lights with long wavelengths pass through the atmosphere

without disturbance because the other colours are scattered. consequently, red and orange lights can be observed. this causes the sun or the sky to appear red or orange at sunset.

7.5 Addition and Subtraction of coloured lights.

Addition of coloured lights.

primary colours are colours which cannot be obtained from mixing other colours.

there are three primary colours, namely red, blue, and green. secondary colours are colours produced by adding primary colours. there are three secondary colours, namely yellow, magenta, and cyan. addition of primary colours and secondary colours and secondary colours

can produce white light. for example, yellow light mixed with blue light will gave white light.

Subtraction of coloured lights. coloured filters consist of primary and secondary filters. primary filters consist of red, green, and blue filters. primary filters only allow lights of the same colours to pass through them,

other coloured lights are absorbed. ex. a green filter only allows green light to pass through it, other coloured

lights are absorbed. secondary filters consist of yellow, magenta and cyan filters. secondary filters allow lights of the same colours and primary colours that

form them to pass through them. ex. a yellow filter allows yellow light and primary colours that form it,

namely red and green lights, to pass through it. the absorption of coloured lights by coloured filters is called subtraction of

coloured lights.

7.5 Principle of Subtraction of Coloured Lights to Explain The Appearance of coloured objects.

the colour of an opaque object depends on the colour of the light that it reflects.

an object of a primary colour, i.e. red, blue or green, only reflects light of the same colour.

for example, a blue object only reflects blue light. an object of a secondary colour, i.e. yellow, magenta, or cyan, reflects

light of the same colour and the primary colours that form it. for example, a magenta object reflects magenta, red, and blue lights.

a white object appears white in white light because a white object reflects all colours. (no coloured lights are absorbed)

a black object appears black because all coloured lights are absorbed by it. (no coloured light is reflected)

absorption of light by coloured objects is based on the principle of subtraction of coloured lights.

the colour of an object depends ona) the colour of the light the strikes itb) the colours of the light absorbedc) the colour of the light reflected

Function of rod cells and cone cells in the eye. the retina contains two types of cell that are sensitive to light

stimulus- rod cells

o is sensitive to light of low intensityo such as nighto not sensitive to colour and only a black and white image

is produced.

- cone cellso is sensitive to light of high intensity to detect colouro there are three types of cone cell, and each is sensitive

to red, green, and blue light respectively.o other colours which can be seen are the products of

mixing the three colours.

Effects of mixing pigments,1. The primary colours of pigments are red, blue, and yellow.2. Mixing red, blue, and yellow in different amounts gives various colours.3. When these colours are mixed in the correct amounts, black is obtained.4. Mixing pigments is based on the principle of subtraction of coloured lights.5. Pigments are not as pure as coloured lights. Pigments have the property of

absorbing and reflecting certain colours when shone on by white light.6. For example,

a. blue pigment reflects violet, blue and green lights.b. magenta pigment reflects red and blue lights.

7. When two pigments are mixed, the colour produced is the colour reflected by both pigments.

8. All colours, except while, can be produced by mixing coloured pigments.

Colour of pigments Colour obtainedRed + Yellow OrangeCyan + Yellow GreenMagenta + Yellow Red

Blue + Yellow GreenMagenta + Yellow + Cyan Black

colours obtained from the mixing some coloured pigments.

7.8 Importance of Colour in Daily Life.

Uses of colour in daily life.The uses of colour in our daily life are as follows : (a) printing (c) traffic lights (b) electrical wiring (d) symbols and lights

Printing colour pictures are printed by using four colours – yellow, magenta, cyan,

and black – on separate plates. the black colour is used to make the picture appears sharp and clearer.

Stage 1 – The image is printed with yellow followed by magenta.Stage 2 – The product of Stage 1 is printed with cyan.Stage 3 – Finally, the picture is printed with black to make the picture clear.

Colour printing the international colour code is used in the wiring of three-pin plugs. live wire is brownish or red in colour neutral wire is blue earth wire is yellow with green stripes or just green in colour

Traffic lights a traffic light consists of red, yellow, and green lamps. each colour represents a certain signal.

Similarity Addition of coloured lights

Mixing coloured pigments

Can be mixed to get other colours

Difference

Impure

Subtraction of colour

Red, blue, yellow

Pure

Addition of colour

Red, blue, green

Purity of colour

Method of colour mixing

Primary colours

Product of colour mixing

red – stop yellow (amber) – ready to stop green – go

Symbols and signals the red light of an ambulance indicates an emergency. the yellow line by the roadside indicates a no-parking area. the white line in the middle of the road separates the left and right lanes of

the road. the two white lines in the middle of the road indicate that no overtaking is

permitted.

Importance of colour to living things. colour is also important to plants and animals. colour enables animals to protect themselves form danger or to make it

easy for them to hunt their preys by means of camouflaging to the surroundings.

some animals have the ability to change the colour of their bodies according to the colour of the surroundings.

the males of many species of birds, like the peacock, have colourful feathers to attract the attention of the females for mating purpose.

colourful flowers attract the attention of insects for the pollination purpose. poisonous animals usually have bright colours to warn other animals. for example, toads that have bright colours are usually poisonous.