Optical Instruments – SS2 Physics Lesson Note (Third Term)

Optical instruments are devices that use lenses or mirrors to help us see objects that are too small, too far away or out of direct view.

Behavioural objectives

1. Describe how a compound microscope forms a magnified image.
2. Describe how an astronomical telescope forms an image and state its disadvantage on Earth.
3. Name the parts of the eye that form an image and state the function of the retina and the iris.
4. Define accommodation and binocular vision.
5. Explain the causes of long sight and short sight.
6. State how each defect is corrected.

What is a compound microscope?

A compound microscope is an instrument with two convex lenses that gives a greater magnification than a simple microscope (a single magnifying glass).

• Use: viewing very small objects, such as cells and micro-organisms.
• Lenses: the objective lens and the eyepiece, both convex.

What are the parts that form the image in a microscope?

• Objective lens: a convex lens of short focal length, placed close to the object.
• Eyepiece: a convex lens used as a magnifying glass to view the image formed by the objective lens.

How does a compound microscope form an image?

1. The object is placed just outside the principal focus of the objective lens.
2. The objective lens forms a real, magnified and inverted image.
3. This image falls within the principal focus of the eyepiece.
4. The eyepiece acts as a magnifying glass and magnifies the image again.
5. The final image is virtual and formed at the least distance of distinct vision from the eye.

What is the magnification of a compound microscope?

The total magnification of a compound microscope is the magnification of the objective multiplied by the magnification of the eyepiece.

• Total magnification: magnification of objective × magnification of eyepiece.
• Least distance of distinct vision: the nearest distance at which the normal eye sees an object clearly, about 25 cm.

What is an astronomical telescope?

An astronomical telescope is an instrument that uses two convex lenses to view distant objects such as stars and planets.

• Construction: it uses two convex lenses, the objective lens and the eyepiece.

What are the parts of an astronomical telescope?

• Objective lens: has a long focal length and forms a real image of a distant object at its focal plane.
• Eyepiece: has a short focal length and magnifies the image formed by the objective.

How does an astronomical telescope form an image (normal adjustment)?

1. Light from a distant object reaches the objective lens as nearly parallel rays.
2. The objective forms a real, inverted image at its focal plane.
3. The lenses are placed so that the principal focus of the objective coincides with the principal focus of the eyepiece.
4. The eyepiece then gives a final image at infinity.

What is the magnification of a telescope in normal adjustment?

In normal adjustment, the magnification of a telescope is the focal length of the objective divided by the focal length of the eyepiece.

• Magnification: focal length of objective ÷ focal length of eyepiece.
• Distance between the lenses: the sum of the two focal lengths.
• Effect: a longer objective focal length or a shorter eyepiece focal length gives a larger magnification.

What is the disadvantage of an astronomical telescope?

The disadvantage of an astronomical telescope is that its final image is inverted (upside down).

• On the stars: this can be tolerated, since stars have no fixed “up” or “down”.
• On the Earth: it is a disadvantage, because objects such as buildings and people appear upside down.

What is the difference between a compound microscope and an astronomical telescope?

Both instruments use two convex lenses, but the microscope views very small near objects and the telescope views very distant objects.

How does the human eye form an image?

The human eye forms a real, inverted image of an external object on the retina.

• Optical system of the eye: the cornea, the aqueous humour, the lens and the vitreous humour.
• Interpretation: the retina sends the impression through the optic nerve to the brain, and the brain interprets it.

What are the parts of the eye and their functions?

What is accommodation?

Accommodation is the ability of the eye to focus on objects at different distances.

• How it works: muscles around the lens change its shape, and so change its focal length.
• Distant object: the lens becomes thinner.
• Near object: the lens becomes thicker.

What is binocular vision?

Binocular vision is seeing with two eyes at the same time.

• Relief: it enables objects to be seen in relief, which gives a sense of depth and solidity.
• Judging distance: it helps a person judge how far away an object is.

What are the common eye defects?

The two common eye defects are long sight (hypermetropia) and short sight (myopia).

What is long sight (hypermetropia)?

Long sight is a defect in which a person sees distant objects clearly but cannot see close objects clearly.

• Effect: the person can see distant objects but cannot see close objects clearly.
• Near point: it is more than 25 cm, the near point of the normal eye.
• Cause: the eyeball is too short, so rays from an object 25 cm from the eye focus behind the retina.
• Correction: a converging (convex) lens placed in front of the eye for near vision.

What is short sight (myopia)?

Short sight is a defect in which a person sees near objects clearly but cannot see distant objects clearly.

• Effect: the person cannot see distant objects clearly, as rays from them focus in front of the retina.
• Far point: it is less than the normal far point, which is at infinity.
• Cause: the eyeball is too long, or the eye lens is too strongly converging.
• Correction: a diverging (concave) lens.
• How the lens works: it makes an object at infinity appear to be at the person’s far point.

What is the difference between long sight and short sight?

Long sight affects near vision and is corrected by a converging lens, while short sight affects distant vision and is corrected by a diverging lens.

Which optical instruments use no lens?

The periscope is the optical instrument in this lesson that uses no lens.

• Periscope: uses plane mirrors (or prisms) to change the direction of light, so it has no lens.
• Projector: uses a lens to form a large image on a screen.
• Microscope and telescope: both use lenses.
• Eye: it has a lens, so it is an optical system with a lens.

Evaluation

1. Name the two lenses of a compound microscope and state which one has a short focal length.
2. Why is the inverted image of an astronomical telescope a disadvantage on the Earth?
3. Which part of the eye regulates the amount of light entering through the pupil?
4. What causes long sight, and how is it corrected?
5. How is short sight corrected?

Assignment

Answer all four questions, choosing the correct option.

1. For correcting long-sight defects in the human eye, we require: (a) converging lens (b) diverging lens (c) microscope (d) periscope.
2. Which of the following optical instruments does not make use of a lens? (a) projector (b) periscope (c) eye (d) microscope.
3. The ability of the eye to focus on objects at different distances is called: (a) power (b) accommodation (c) normal vision (d) long sight.
4. Binocular vision: (a) restricts the field of view (b) enables a person to see further (c) enables objects to be seen in relief (d) enables objects to be seen clearly.