Wonders of Light – Part I – Maharashtra Board Class 10 Solutions for Science and Technology (English Medium)
- A ray of light parallel to the principal axis, after reflection from a concave mirror, passes through the focus of the mirror.
- The focal length of a convex lens is positive.
- The image of an object is formed behind the retina in hypermetropia.
- An optical device used by watch repairers is a simple microscope.
- The power of spectacle for myopic eye is negative.
- In torches, the source of light is placed at the focus of the concave mirror to obtain a parallel beam of light.
- In projector lamps, the source of light is placed at the centre of curvature of the concave mirror.
- In floodlights, the source of light is placed just beyond the centre of curvature of the concave mirror.
Rules for drawing ray diagrams for the formation of image by a spherical mirror:
- If the incident ray is parallel to the principal axis, then the reflected ray passes through the focus of a concave mirror or appears to diverge from the focus of a convex mirror when extended backwards.
- If the incident ray passes through the focus of a concave mirror or is directed towards the focus of a convex mirror, then the reflected ray is parallel to the principal axis.
- If the incident ray passes through the centre of curvature of a concave mirror or is directed towards the centre of curvature of a convex mirror, then the reflected ray traces the same path.
The ability of an eye lens to adjust its focal length is called the power of accommodation of eye.
It is done by changing the curvature of the elastic eye lens by making it thinner or thicker.
- Iris: The coloured part of the eye is the iris which helps regulate the amount of light entering the eye. When there is bright light, the iris closes the pupil to let in less light, while when the light is less, the iris opens the pupil to let in more light.
- Ciliary muscle: The ciliary muscles hold the eye in its position. It changes the curvature of the eye lens which alters the focal length of it, so that the image is formed on the retina of the eye.
The given part of a hollow spherical glass can be converted into a concave mirror by
1. polishing its inner side to make it reflecting or
2. coating its outer side with a thin layer of silver and painting it with red colour to protect the silver coating.
The distance between the screen and a person in a cinema hall is less, so the intensity of light falling on the eye is more.
Because of this, there is great contraction of the pupil of the eye which causes a strain. Hence, you cannot enjoy watching a movie from a very short distance from the screen in a cinema hall.
- When an object is placed within the focal length of a simple microscope, its magnified and erect image is obtained on the same side of the lens as that of the object.
- So, a watch repairer sees the minute parts of a watch very clearly with a simple microscope by adjusting the distance between the object and the lens. This helps in repairing the watch. Hence, a simple microscope is used by watch repairers.
A person suffering from myopia sees nearby objects clearly but cannot see distant objects clearly as the image of a distant object falls in front of the retina.
When lenses of appropriate power are used, the parallel rays coming from a distant object are diverged to the proper extent before they enter the eye lens, as the concave lens is a diverging lens. So, after the converging action of the eye lens, the image of the distant object is pushed back and it falls on the retina. Thus, the object at a distance can be seen clearly and the defect is eliminated.
- The power of accommodation of the eye usually decreases with ageing. So, an elderly person finds it difficult to see nearby objects clearly. This can be corrected by using a suitable convex lens.
- At the same time, an aged person may not be able to see distant objects clearly due to low converging power of the eye lens. It is because of the weak action of ciliary muscles and/or shortening of the eye ball or flattening of the eye lens. This defect can be corrected by using a suitable concave lens. Thus, for such a person both lenses are required. Hence, a bifocal lens is necessary.
In solar devices, sunlight is used for energy. The Sun is a distant object, so the rays coming from it are parallel to the principal axis and are incident on a concave mirror. Then after reflection, they converge to a single point. So, the heat is concentrated at that point which gives better results. Hence, concave mirrors are used in solar devices.
- The defect shown in the figure is hypermetropia or long sightedness.
- Two possible reasons are
- Ciliary muscles have become weak, so the converging power of the eye lens decreases.
- Due to shortening of the eye ball or flattening of the eye lens.
3. To correct the eye defect, a suitable convex lens is used.