A Galilean Telescope or a terrestrial telescope is an optical instrument which is used to see the magnified image of distant objects on earth. Galilean telescope consists of a convex lens as objective lens but a concave lens as eye piece. It always forms erect images of the objects seen through it.

Differences between Astronomical Telescope and Galilean Telescope

Test Your Understanding and Answer These Questions:

1. Which instrument is used to see distant objects?
2. Why Galilean telescope is not used to seen heavenly bodies?
3. Give differences between astronomical and Galilean telescope.

A telescope is an optical instrument which is used to see the distant objects. Telescopes are of two types:

    1. Astronomical telescope
2. Galilean telescope or Terrestrial telescope

Astronomical Telescope

An astronomical telescope is an optical instrument which is used to see the magnified image of distant heavenly bodies like stars, planets, satellites and galaxies etc.

The final image formed by an astronomical telescope is always virtual, inverted and magnified.

Principle of Astronomical Telescope

An astronomical telescope works on the principle that when an object to be magnified is placed at a large distance from the objective lens of telescope, a virtual, inverted and magnified image of the object is formed at the least distance of distinct vision from the eye held close to the eye piece.

Construction of Astronomical Telescope

An astronomical telescope consists of two convex lenses : an objective lens O and an eye piece E. the focal length fo of the objective lens of astronomical telescope is large as compared to the focal length fe of the eye piece. And the aperture of objective lens O is large as compared to that of eye piece, so that it can receive more light from the distant object and form a bright image of the distant object. Both the objective lens and the eye piece are fitted at the free ends of two sliding tubes, at a suitable distance from each other.

Working of Astronomical telescope

The ray diagram to show the working of the astronomical telescope is shown in figure. A parallel beam of light from a heavenly body such as stars, planets or satellites fall on the objective lens of the telescope. The objective lens forms a real, inverted and diminished image A’B’ of the heavenly body. This image (A’B’) now acts as an object for the eye piece E, whose position is adjusted so that the image lies between the focus fe’ and the optical centre C₂ of the eye piece. Now the eye piece forms a virtual, inverted and highly magnified image of object at infinity. When the final image of an object is formed at infinity, the telescope is said to be in ‘normal adjustment’.

It should be noted that, the final image of object (such as stars, planets or satellites) formed by an astronomical telescope is always inverted with respect to the object. But it does not matter whether the image formed by an astronomical telescope is inverted or not, as all the heavenly bodies are usually spherical is shape.

Magnifying Power of an Astronomical Telescope

The Magnifying Power of a telescope is given by:

m    =    

Where, fo = Focal length of the objective lens
fe = Focal length of the eye-piece lens

And the length (L) of the tube of telescope is equal to the sum of the focal lengths of the objective lens and the eye piece. Thus,

L = fo + fe

Test Your Understanding and Answer These Questions:

1. Define telescope.
2. Name the types of telescope.
3. What is an astronomical telescope?
4. Explain principle, construction and working of an astronomical telescope with the help of a ray diagram.
5. What is magnification of astronomical telescope?
6. Which lens has small focal length in astronomical telescope?
7. How many lenses are used in a telescope?
8. Which instrument is used to see heavenly bodies?

A compound microscope is an optical instrument consisting of two convex lenses of short focal lengths which is used for observing the highly magnified images of tiny objects. The compound microscope can magnify the image of a tiny object up to 1000.

Principle of compound microscope

A compound microscope works on the principle that when a tiny object to be magnified is placed just beyond the focus of its objective lens, a virtual, inverted and highly magnified image of the object is formed at the least distance of distinct vision from the eye held close to the eye piece.

Construction of compound microscope

A compound microscope consists of two convex lenses: an objective lens O of small aperture and an eye piece E of large aperture. The lens which is placed towards the object is called objective lens, while the lens which is towards our eye is called eye piece. These two convex lenses i.e. the objective and the eye piece have short focal length and are fitted at the free ends of two sliding tubes at a suitable distance from each other. Although the focal length of both the objective lens and eye piece is short, but the focal length of the objective lens O is a little shorter than that of the eye piece E.

The reason for using the eye piece of large focal length and large aperture in a compound microscope is, so that it may receive more light rays from the object to be magnified and form a bright image.

Working of compound microscope

The ray diagram to show the working of compound microscope is shown in figure. A tiny object AB to be magnified is placed in front of the objective lens just beyond its principal focus fo’. In this case, the objective lens O of the compound microscope forms a real, inverted and enlarged image A’B’ of the object.

Now A’B’ acts as an object for the eye piece E, whose position is adjusted so that A’B’ lies between optical centre C2 and the focus fe’ of eye piece. Now the eye piece forms a final virtual, inverted and highly magnified image A”B”. this final image A”B” is seen by our eye hold close to eye piece, after adjusting the final image A”B” at the least distance of distinct vision of 25 cm from the eye.

 Magnification of compound microscope

 The magnification of compound microscope is given by:

m    =    

where,   D = Least distance of distinct vision (25 cm)
L = Length of the microscope tube
fo = Focal length of the objective lens
fe = Focal length of the eye-piece lens

Differences between Simple Microscope and Compound Microscope

Test Your Understanding and Answer These Questions:

1. What is a compound microscope?
2. Define objective lens.
3. Define eye piece.
4. How many lenses are used in compound microscope?
5. What is magnification of compound microscope?
6. Explain the principle, construction and working of compound microscope with the help of a diagram.
7. Give differences between simple microscope and compound microscope.

A microscope is an optical instrument which is used to see highly magnified images of tiny objects such as bacteria, cells , viruses and protozoans etc. because these tiny objects cannot be seen by naked eyes. In this chapter we shall study two types of microscopes:

1. Simple microscope
2. Compound microscope

Simple Microscope

A simple microscope is also called magnifying glass. It is actually a convex lens of small focal length, which is used for seeing the magnified images of small objects.

Principle of Simple Microscope

A simple microscope works on the principle that when a tiny object is placed within its focus, a virtual, erect and magnified image of the object is formed at the least distance of distinct vision from the eye held close to the lens.

Working of Simple Microscope

The ray diagram to show the working of simple microscope is shown in figure. A small object AB which is to be magnified is placed between the principal focus F’ and optical centre C of the convex lens. Now, a ray of light AO parallel to principal axis which is coming from the point A of the object passes through the focus F along the straight line OX after getting refracted by the convex lens. A second ray of light AC coming from the point A of the object passes through the optical centre C of the convex lens along the straight line CY. As is clear from the figure that the two rays i.e. OX and CY are diverging rays so these rays can intersect each other only at point A’ when produced backward. Now, on drawing A’B’ perpendicular from point A’ to the principal axis, we get the image A’B’ of the object which is virtual, erect and magnified.

Uses of Simple Microscope

Following are the important uses of simple microscope:

1. The simple microscope is commonly used by watch makers to see the magnified view of small parts of a watch.
2. It is also used by the jewelers to see the magnified view of the fine parts of jewellery.
3. Simple microscope is used to see the enlarged image of letters of a book, textures of fibers or threads of a cloth.
4. Simple microscope is used to see the magnified view of different particles of different types of soils.
5. It is used by palmists to see enlarged view of the lines of our hand.
6. Simple microscope is used by skin specialists to find out various diseases of skin.
7. It is also used to see the details of stamp and engravings.

Magnification of Simple Microscope

The magnifying power of a simple microscope is given by:

m    =     1    +    

Where, D = least distance of distinct vision
F = focal length of the convex lens

It should be noted that the focal length of the convex lens should be small because smaller the focal length of the lens, greater will be its magnifying power. Also the maximum magnification of a simple microscope is about 10, which means that the object will appear 10 times larger by using the simple microscope of maximum magnification.

Test Your Understanding and Answer These Questions:

1. Define microscope.
2. Name types of microscope.
3. What is a simple microscope?
4. Give uses of simple microscope.
5. Which type of lens is used as a simple microscope?
6. Explain the principle, construction and working of simple microscope with the help of a ray diagram.
7. Explain magnification of a simple microscope. How it can be increased?
8. The focal length of lens used in a simple microscope should be small. Why?

Cataract

Cataract is usually found in old persons. Cataract causes the loss of eye sight due to the formation of an opaque membrane over the eye lens.

 Correction of Cataract

This defect cannot be corrected by using any type of spectacles. The only method for its correction is the removal of opaque lens by cataract surgery.

Presbyopia or Old Age Longsightedness

Due to Presbyopia an old person cannot see nearby objects clearly. That is why he feels difficulty in reading and writing. This defect occurs due to the loss of elasticity of eye lens and gradual weakening of cilliary muscles.

Correction of Presbyopia

This defect is also corrected by using spectacles having suitable convex lens.

Astigmatism

Due to astigmatism blurred images are formed due to irregularities in the curvature of cornea. Due to this defect, the light rays coming from an object are focused at different points.

Correction of Astigmatism

This defect is corrected by using cylindrical lenses.

Test Your Understanding and Answer These Questions:

1. What is cataract?
2. How can cataract be cured?
3. What is presbyopia?
4. How can presbyopia be corrected?
5. What is astigmatism?
6. How can astigmatism be corrected?

Due to Hypermetropia the eyes are able to see the distant objects clearly but cannot see the nearby objects clearly. In this case the image of nearby object is formed beyond the retina.

Cause of Hypermetropia

An eye suffers from Hypermetropia due to the following two reasons

1. Abnormally short eye ball
2. Abnormally low convexity of the eye lens

Correction of Hypermetropia

This defect is corrected by using spectacles having suitable convex lenses. The convex lens converge the light rays, so the image is formed on the retina.

Test Your Understanding and Answer These Questions:

1. What is hypermetropia?
2. How is hypermetropia caused?
3. How can hypermetropia be corrected?
4. Which type of lens is used to correct hypermetropia?

Due to myopia the eyes are able to see nearby objects clearly but cannot see the distant objects clearly. This defect is caused due to the formation of image of distant object in front of the retina.

 

Cause of Myopia

An eye suffers from the defect of myopia due to the following two reasons

1. Abnormally long eye ball
2. Abnormally high convexity of eye lens

Correction of Myopia

This defect is corrected by using spectacles having suitable concave lenses. The concave lenses diverge the light rays, so the image is formed exactly on the retina.

Test Your Understanding and Answer These Questions:

1. What is myopia?
2. How is myopia caused?
3. How can myopia be corrected?
4. Which type of lens is used to correct myopia?

Human eye is very sensitive organ of the body. It is essential for every individual to take full care for keeping human eye healthy. There are many types of defects that develop in human eye. Following are the common defects found in a human eye:

• Myopia or nearsightedness
• Hypermetropia or longsightedness
• Cataract
• Presbyopia or old age longsightedness
• Astigmatism

Test Your Understanding and Answer These Questions:

1. What are the common defects of human eye?

Retina contains a large number of light sensitive cells called rod cells and cone cells. The rod cells respond only to the intensity of light, while the cone cells respond to the colours of an object. There are 3 types of cone cells which are sensitive to red, blue and green colours to different extent i.e. when a red light falls on the retina then it mainly stimulates the red sensitive cone cells. These cone cells then send the information to brain in the form of electric signals through optic nerve. This gives rise to the sensation of that particular colour in our brain.

Sometimes, some particular types of cone cells are absent in a person due to which that person cannot see that particular colour. This condition is called colourblindness. Colourblindness may be defined as the inability of a person to distinguish between certain colours. For example, if a person doesn’t possess red-green sensitive cone cells then he cannot distinguish these colours from other colours. Colour blindness is a hereditary or genetic disorder which is not curable.

Test Your Understanding and Answer These Questions:

1. How do we see colours?
2. What is colourblindness?
3. What is significance of rod cells and cone cells in human eye.

We have already studied that the amount of light entering the eye through pupil is controlled by iris. But this adjustment takes some time. For example, our eyes cannot see everything clearly when we enter a dark room from a bright light. The reason for this is that in bright light the size of pupil of our eye is small. When we enter the dark room, due to small size of pupil, very little light enters our eyes. But after some time, the pupil expands and we become able to see things as more light enter our eyes now.

In the same way, while coming out from a dark room to bright light, our eyes feel a glaring affect. This is due to the reason that in dark room the pupil is large so when we come out in bright light, a large amount of light enters our eyes and we feel glare. But after sometime, the pupil contracts and we become able to see clearly as less light enters our eyes now.

Test Your Understanding and Answer These Questions:

1. What is dark and light adaptation?
2. What happens to our eyes when we enter a dark room from bright sunlight?
3. What happens to our eyes when we go out suddenly in bright light from a dark room?