L. R. & S. M. VISSANJI ACADEMY SECONDARY SECTION - 2016-17 PHYSICS-GRADE: VIII OPTICAL INSTRUMENTS SIMPLE MICROSCOPE A simple microscope consists of a single convex lens of a short focal length. The object to be seen is placed between the focus and the optical centre of the lens and the image of the object formed is virtual, enlarged and on the same side as that of the object. A magnifying glass is an example of a simple microscope. A magnifying glass/simple microscope is used for the following reasons: For reading very small letters. A doctor uses it to examine the skin of a patient. A watch repairer uses it so that an enlarged image of the smaller parts of the watch may be seen clearly. We can see that the ray AP from A, parallel to the principal axis passes through the focus after refraction through the lens and another ray from A passing through the optical centre gets refracted without deviation. When the two refracted rays falls on the eyes of the observer, it appears to diverge from point A. Extending the rays in the backward direction, we get A B as the image of the object AB. The eye thus sees the magnified image A B. CAMERA A photographic camera is an optical instrument used to record permanent images of an object on a photographic film. It consists of a convex lens fitted in one face of a light proof box which is painted black from inside. A strip of photographic film is placed on the other ace of the box. Working: The object to be photographed and the camera lens has to be kept at a distance greater than twice the focal length of the lens. To take a photograph, the camera lens is aimed at the object. A sharp and clear image of the object is obtained on the film by changing the distance between the camera lens and the film. This is called focusing. To focus the distant object on the film, we have to decrease the distance between the camera lens and the film. To focus the nearby objects, we have to increase the distance between the camera lens and the film. On clicking, the shutter of the camera (which is placed just behind the lens) opens for a fraction of a second. Light from the object enters the camera through the lens and the opened shutter, and falls on the light-sensitive photographic film. An image of the
object is formed on the film. Then the exposed film, or negative is developed to obtain the actual photograph. COMPOUND MICROSCOPE The compound microscope was invented by Anton van Leeuwenhoek. A compound microscope is a combination of two convex lenses- the objective lens and the eyepiece. The objective is a lens with a small focal length which is placed close to the object. The eyepiece is a lens which has a large focal length. The eye is placed near the eyepiece. Working: A well illuminated small object is placed between F and 2F of the objective lens. A real, inverted and magnified image is formed on the other side of the objective lens. This real image acts as the object for the eyepiece lens. The eyepiece lens is so adjusted that the image formed by the objective lens falls within the focus and optical centre of the eyepiece. Therefore, a virtual, highly magnified final image is formed by the eyepiece lens. This final image is inverted with respect to the object. TELESCOPE A telescope is an optical instrument which is used to obtain an enlarged view of distant objects. An astronomical telescope is used to have close-up views of celestial bodies like the moon and the planets.
A telescope consists of two convex lenses placed at either ends of a long tube. The convex lens facing the object is called the objective lens and has a large focal length. The focal length of the objective is taken as large as possible so that more light from a distant object can be collected by it. The convex lens closer to the eye is called the eyepiece lens. The eyepiece lens of a telescope has a small focal length. Principle of telescope: A real, inverted and highly diminished image of the distant object is formed by the convex lens which is the objective lens. If this image falls within the focal length of another convex lens, an enlarged erect and virtual image of this imge is formed. Working: When parallel rays from a distant object are incident on a convex lens (objective lens); a real inverted and highly diminished image is formed between the focus and centre of curvature (i.e. F o and 2F o ) of the objective lens. The image thus formed acts as the object for the eyepiece lens and it is formed between the focus and the optical centre of the eyepiece lens. Since the object for the eyepiece lens is between the focus and the optical centre (i.e. F and O) of convex lens (eyepiece lens), an inverted ( w.r.t. the object ), enlarged and virtual image is formed. Here the eyepiece acts as a simple microscope. The final image is much smaller than the object itself but it is much larger than the object as seen by the naked eye. HUMAN EYE The human eye is the most important sense organ and the most remarkable optical instrument. The eye has a roughly spherical shape. The outer covering of the human eye from inside called as the sclera is white in colour. Its transparent front part is called cornea. Behind the cornea, there is a dark muscular structure called iris. In the iris, there is a small opening called the pupil. The size of the pupil is controlled by the iris. The iris controls the amount of light entering the eye. Behind the pupil of the eye is a convex lens. The lens focuses light on the back of the eye on a layer called retina. The retina contains several nerve cells. At the junction of the optic nerve and the retina, there are no sensory cells, so no vision is possible at that spot. This is called the blind spot. A jelly like substance called vitreous humour fills the space between the lens and the retina. The space between the lens and the cornea is filled by a clear fluid called aqueous humour. The lens is held in place by the suspensory ligaments.
POWER OF ACCOMMODATION OF AN EYE The ability of the eye lens to change the power of the lens to accommodate the near and far off distances on the retina is called the power of accommodation of the eye. The power of accommodation takes place by the contraction and relaxation of the ciliary muscles. When a distant object is to be seen, the curvature of the lens decreases, thereby increasing its focal length and in case of nearby objects, it is vice-versa. Thus, by changing the curvature of the lens and its focal length, the image of the nearby or distant objects can be brought in focus on the retina. LEAST DISTANCE OF DISTINCT VISION Normal eyes can see distant as well as near objects clearly. This is due to the ability of the ciliary muscles to change the focal length of the eye lens. The least distance at which the normal eye can see an object clearly without strain is called the least distance of distinct vision which is 25 cm. DEFECTS OF VISION In a healthy eye, the muscles surrounding the lens automatically adjust the curvature of the lens which enables it to form sharp image of the object on the retina irrespective of the distance of the object from the eye. Sometimes, the curvature of the lens cannot be adjusted to the required value for the formation of sharp images for some reason. This is called the defects of vision. There are two types of defects in the human eye: Myopia or short-sightedness. A person suffering from this defect can see nearby objects clearly but cannot see the faroff objects clearly. Far objects cannot be seen because the images of these objects are formed in front of the retina. This defect of vision can be rectified by using spectacles with concave lens of appropriate focal length. When a concave lens is placed in front of the defective eyes, the rays coming from an object at infinity are diverged by the concave lens and the image of the object gets formed on the retina.
This defect is usually found in young people. Hypermetropia or long-sightedness. A person suffering from hypermetropia can see far-off objects clearly,but cannot see nearby objects clearly. Nearby objects cannot be seen because the images of these objects are formed not on the retina, but behind the retina. This defect of vision can be corrected using spectacles with convex lens of appropriate focal length. When a convex lens is placed in front of the defective eyes, the rays coming from the nearby distance appear to come from a point which is the near point of the defective eye (which is at a distance of 25 cm) due to which the image forms on the retina and the object can be seen clearly. This defect is usually found in old people.
ONE SIMILARITY AND DISSIMILARITY BETWEEN AN EYE AND A CAMERA. Similarity: The image formed by an eye as well as a camera is real. Inverted and diminished. Dissimilarity: The image formed by an eye is temporary whereas the image formed by a camera is permanent. CHARACTERISTICS OF THE FINAL IMAGE FORMED BY: Magnifying glass - The image formed by a magnifying glass is erect, virtual and magnified and it is formed on the same side as that of the object. Telescope - The final image is virtual and smaller than the object itself, but it is much larger than the object as seen by the naked eye. Compound microscope The final image is inverted with respect to the object and is highly magnified as compared to the object. COMPARISON BETWEEN THE FOCUSING IN A CAMERA AND THE FOCUSING IN A HUMAN EYE. The human eye is a wonderful instrument, relying on refraction and lenses to form images. There are many similarities between the human eye and a camera, including: a diaphragm to control the amount of light that gets through to the lens. This is the shutter in a camera, and the pupil, at the center of the iris, in the human eye. a lens to focus the light and create an image. The image is real and inverted. a method of sensing the image. In a camera, film is used to record the image; in the eye, the image is focused on the retina, and a system of rods and cones is the front end of an image-processing system that converts the image to electrical impulses and sends the information along the optic nerve to the brain. With a camera, the lens has a fixed focal length. If the object distance is changed, the image distance (the distance between the lens and the film) is adjusted by moving the lens. This can't be done with the human eye: the image distance, the distance between the lens and the retina, is fixed. If the object distance is changed (i.e., the eye is trying to focus objects that are at different distances), then the focal length of the eye is adjusted to create a sharp image. This is done by changing the shape of the lens; a muscle known as the ciliary muscle does this job.