COLOURS OF OBJECTS
Colour may be defined as the visual sensation which depends upon the wavelength or frequency of the
light that enters the eye.
Primary and Secondary Colors
1. Red,
2. Green
3. Blue
A color resulting from the mixing of two primary colors are called secondary or composite colors or
subtractive primaries.
Mixing of Colors
Red + Blue = Magenta
Blue + Green = Cyan (Peacock blue)
Red + Green = Yellow
Red + Green + Blue = White
COMPLEMENTARY COLORS
A pair of colors which on mixing produce white are called complementary colors.
Colour of object on which light Reflected ray (which one sees) Absorbed rays
is incident
Red Red Green, blue
Blue Blue Red, green
Green Green Red, blue
Cyan Green, blue Red
Magenta Red, blue Green
Yellow Red, green Blue
COLOUR MIXING PIGMENTS
In colour mixing by subtraction, is observed that when different pigments are mixed, e.g. when yellow
and blue paints are mixed, the resulting paint is green in color because yellow paint absorbs blue and
violet and the blue absorbs red and yellow, leaving green as the only colour that the mixture could
reflect.
OPTICAL INSTRUMENTS
CAMERA
The camera is used in taking photographs of an object. It consists of a light proof box with a
converging lens in front and a light sensitive film at the back.
It consists of the following:
- Aperture: It is a hole through which rays enter the camera and the size of the hole is controlled
by the adjustable diaphragm.
- The shutter controls the time interval during which light is allowed to fall on the film. [time
interval/exposure time]. A shutter is placed between the lens and the film. When a photograph
is taken, the shutter opens and closes quickly, thereby exposing the film to light for a short time
to light entering the camera.
- The sensitive photographic film is very sensitive to light. A sharp image of the object is focused
on the film by adjusting the distance of the lens from the film.
- A focusing ring, by means of this ring, the distance between the lens and the film can be
adjusted by moving the lens forward and backward.
- The diaphragm controls the amount of light entering the camera.
THE HUMAN EYE
The essential part of human eye:
1. The cornea:- it serves as a protective converging in front of the lens and allowed light into the
eye.
2. The aqueous humour:- It serves mainly to keep the eye in its spherical shape.
3. The iris serves as the diaphragm. It regulates the amount of light entering the eye.
4. The ciliary muscles:- Contraction and expansion of ciliary muscles alters the focal length of the
lens.
5. The retina is the light sensitive area of the cells at the back of the eye.
The far point of the eye is the furthest distance at which the eye can focus an object. This is infinity for
normal eye.
The near point of the eye is the minimum distance of distinct vision. For a normal eye, this is 25 cm.
Accommodation
Accommodation is the ability of the eye to alter or adjust the focal length of its lens so as to form clear
images of objects at different distances on its retina.
Binocular Vision
Binocular Vision is the overlapping of the two images formed by both eyes which gives an impression of
depth and solidity and makes it possible for us to see things relief.
Persistence Vision:
The persistence of vision is the persistence of sensation of vision even after light from an object has
been switched off.
Eye Defects and Correction
Eye defect is a disease of the eye which is experienced when light refraction coming from the object
through the lens is not properly focused on the retina. Some of the eye defects are:
1. Myopia (Short Sightedness): A short sightedness is an eye defect in which one can only see
object placed near the eye but cannot see clearly object far from the eye. This arises when the
eye balls are two long or the crystalline lens is two converging that rays from distant objects are
brought to a focus in front of the retina instead of at the retina. Its far point is less than the
normal far point which is at infinity.
Short sightedness can be corrected by placing suitable concave or diverging lens in front of the
eye.
2. Hypermetropia (Long sightedness): Long sightedness is an eye defect in which one can only see
long distance objects clearly. This arises as a result of eye balls being too short or crystalline lens
not being sufficiently convergent.
Rays from near objects are brought to a focus behind the retina. Its near point is farther away
than 25 cm, the normal near point.
Long sightedness can be corrected with the aid of a suitable converging lens or convex lens
placed in front of the eye
3. Astigmatism
This is an eye defect caused by the cornea. The rays of objects are reflected at different rates
because of the roughness, muteness and thickness of the cornea and the lens. Astigmatism is
the inability to see things more clearly in one direction than in other direction. The defect is
corrected by placing a suitable cylindrical lens.
4. Presbyopia
This is an eye defect in which the eye is unable to accommodate. It is caused by the inelasticity
of the eye lens due to old age. Bi-focal lens is used to correct Presbyopia.
Differences between Human eye and camera lens
Human eye Camera
1 It has variable focal length It has fixed focal length
2 It has fixed image distance/lens It has variable image distance/lens
3 Its position does not change Its position can be changed
4 It can suffer from defects of vision It cannot suffer from defects of vision
5 It is a biological organ The camera is a mechanical device.
Similarities between human eye and camera
Human eye Camera
1 The eye iris controls the amount of light The diaghram controls the amount of light entering
entering the eye the camera
2 The pupil serves as the passage for light The aperture serve as the passage for light into
into the eye camera
3 The interior is black and light proof to The interior is also black and light proof to prevent
prevent reflection of stray rays of light the reflection of stray rays of light.
4 It works on the process of refraction It works on the process of refraction
5 The retina is the most light sensitive part The photographic film is the most light sensitive
part.
6 It has converging lens to focus rays from It has converging lens to focus rays from an
an external object external object.
Example:
1. A short-sighted person cannot see distinctly objects beyond 80 cm from his eye. What is the
focal length of the correcting lens needs to see distant objects clearly?
Solution
v=−80 cm u=∞
1 1 1
= +
f u v
1 1 1
= + , ∞=0
f ∞ −80
f =−80 cm
Therefore, a concave lens of focal length 80 cm is required.
2. A certain far-sighted person cannot see clearly objects closer to the eye than 75 cm. Determine
the power of the spectacle lenses which will enable her to read type at a distance of 25 cm.
Solution
v=−75 cm u=25 cm
1 1 1
= +
f u v
1 1 1 2
= + =
f 25 −75 75
f =37.5 cm=+ 0.375 m
1 1
power= = =+ 2.7 dioptre
f 0.375
The person needs convex lens of power + 2.7 dioptres.
PROJECTOR
Projector is an instrument used in projecting the image of a transparent slide or other object on to a
screen.
The essential parts are:
(i) Power source
(ii) Condenser
(iii) Slide carrier
(iv) Focusing lens (converging lens)
(v) White screen
