Science [2 Mark Questions]

Concave mirrors are used as shaving mirrors. This is because when the face is placed close to a concave mirror (so that the face is within its focus) the concave mirror produces a magnified and erect image of the face. Since a large image of the face is seen in the concave mirror, it becomes easier to make a smooth shave.

When the object is at a considerable distance (or at infinity) from a concave mirror, then its image is formed at the focus. This fact can be used to find out the focal length of a concave mirror quickly but approximately.We focus a distant object (several metres away) like a window or tree on a screen by using a concave mirror whose focal length is to be determined. The sharp image of window or tree will be formed at the focus of the concave mirror. That is, the distance of image (or screen) from the concave mirror will be equal to the focal length of concave mirror. This distance can be measured with a scale. It will give us the approximate focal length of the concave mirror.

A image of our face in a plane mirror is laterally inverted, so left is right and right is left. However, in a photograph of our face this is not the case. A mirror surface is smooth, so the reflection by a mirror is a regular reflection. A parallel beam of light incident on it, gets scattered by making reflected rays in different directions.

Real image can be obtained on a screen because light rays actually pass through a real image but virtual image cannot be formed on screen because light rays do not actually pass through a virtual image.
Example:
The image formed on a cinema screen is an example of real image.
The image formed by a plane mirror is a virtual image.

Convex mirrors give a virtual, erect, and diminished image of the objects placed in front of them. They are preferred as a rear-view mirror in vehicles because they give a wider field of view, which allows the driver to see most of the traffic behind him.

The image will larger than the object.

When an object is placed at the centre of curvature (C) of a concave mirror, the image formed is:

1. At the centre of curvature (C).
2. Real and inverted.
3. Of same size as the object.

1.  

2. Lateral inversion.

Two uses of convex mirror:

1. A convex mirror is used as a rear-view mirror in vehicles because it forms erect and dimished images of the objects and has a wider field of view.
2. Big convex mirrors are used as security mirror in shops so that a large number of goods displayed in the shop can be seen in the convex mirror.

1. Mirror B is convex, as it forms a small image.
2. Mirror A is concave, as it forms a large image.

Formation of real magnified image by a convex lens.

Convex lens is used.

The power of the lens is the reciprocal of the focal length of the lens. The SI unit of the power is diopter. it is denoted by D.

A wall has a rough surface, so the reflection by a wall is a diffuse reflection. A parallel beam of light incident on it is reflected in different directions.

All the light rays which are parallel to the principal axis of a concave mirror, converge at the the principal focus (F) after reflection from the mirror. Since a concave mirror converges a parallel beam of light rays, it is also called a converging mirror.

Unit of power of a lens is dioptre. One dioptre is the power of a lens whose focal length is 1 metre.

If n decreases, f increases. So if you immerse the convex lens in a liquid whose refractive index is greater than refractive index of material of lens then it will change its nature and become concave lens and thus it will start diverging the rays rather than focusing them on a single point.

When an object is placed beyond the centre of curvature (C) of a concave mirror, the image is:

1. Between the focus and centre of curvature.
2. Real and inverted.
3. Smaller than the object (or diminished).

Concave lens is used.

Concave lens, like any other lens works on the principle of refraction. We can divide a lens into tiny pieces which resemble a triangular prism. When a ray of light falls on it, due to the shape of the lens piece, the light refracts either inwards our outwards. Concave lens is such that it is thicker outside and thinner inside (opposite to that of a convex lens), so the light rays are refracted outwards and thus they diverge.

A concave lens always forms the virtual image of an object.

v = + 50 cm
Since image is real and of same size. The position of image should be double the focal length.
Hence, the object should be at 2f.
V = 2f = 50, f = 25 cm.
Power = 1/f = 100/25 = 4D

1. The image will be virtual and magnified. We know that a convex lens forms the image of the same size as that of the object when the object is placed at 2f. Thus, the focal length of the lens is 13cm. In this case, the object is placed between the focus​(f) and the optic centre. This position of the object results in the formation of a virtual and magnified image.
2. The image will be real and magnified because the object is placed between 2f and f.

The ratio of the focal lengths of mirrors AB and CD will be equal to one as both the mirrors are part of same spherical ball. Hence, their radius of curvature and focal length will be same. The mirror AB will always form a virtual image when object is placed infront of it because mirror AB is a conex mirror.

Coming Soon

Both, concave lens and convex mirror, diverge parallel rays of light coming from infinity (parallel to the principal axis).

Concave mirror is used in a solar furnace. The solar furnace is placed at the focus of a large concave reflector. When parallel rays of light from the sun fall on the surface of the concave mirror, rays gets reflected and meet at the focus of the mirror due to the converging nature of concave mirror. Thus, the furnace kept at the focus becomes very hot. Even steel can be melted in this furnace.

Power of a lens is a measure of the degree of convergence or divergence of light rays falling in it. Power of a lens depends of its focal length.

The lens shown in convex. the parallel rays will converge to a point called focus (F).

To determine the focal length of a convex lens, we put the convex lens in a holder and keep it in front of a distant object like a window or tree, so that the rays coming from the window pass through it. A cardboard screen is put behind the lens. We change the distance of the screen from the convex lens until a clear inverted image of the window is formed on the screen. Measure the distance of the screen from the lens with a scale. This distance will be the focal length of convex lens.

Concave mirror is used in the headlights of a car. This is because when a lighted bulb is placed at the focus of a concave mirror reflector, then the diverging light rays of the bulb are collected by the concave reflector and then reflected to produce a strong, parallel-sided beam of light (which travels a considerable distance in the darkness of night).

Laws of refraction,

First law: According to the first law of refraction, the incident ray, the refracted ray and the normal at the point of incidence, all lie in the same.
Second law: According to the second law of refraction, the ratio of the angle of incidence to the point of incidence, all lie in the same plane.

1. Principal focus of a concave mirror: The principal focus of a concave mirror is a point on its principal axis to which all the light rays which are parallel and close to the axis, converge after reflection from the concave mirror.
2. Focal length of a concave mirror: The focal length of a concave mirror is the distance between its pole and the principal focus.

A communications satellite sends a parallel beam of signals down to the Earth. If these signals are to be focussed onto a small receiving aerial, then the best shape of the metal dish that can be used to collect them would be concave. The metal dish will behave like a concave mirror, which is a converging mirror.

When a beam of light enters glass at an angle, the speed changes and therefore the direction of light changes i.e., bending of light occurs. When a beam of light falls at right angles to the surface of glass, all parts of light waves reach the glass at the same time, enter the glass at the same time and hence slow down at the same time. Due to this no change in direction of light takes place i.e., bending of light does not take place.

Two types of spherical mirrors are:

1. Concave mirror.
2. Convex mirror.

Type of mirror represented by the:

1. Back side of a shining steel spoon – convex mirror.
2. Front side of a shining steel spoon – concave mirror.

When the boy walks towards the mirror at a speed of 1m/ s, his image will also appear to move towards the mirror at the same speed of 1m/ s. So, the speed at which his image approach him will be 2m/ s + 2m/ s = 4m/ s.

1. Light speeds up as it travels from a denser medium to a rarer one, ie., from a medium with a higher refractive index to one with a lower refractive index. An example would be the medium pair of diamond and water, where light travels from diamond to water.
2. Using the same argument, light slows down as it travels from a medium with a lower refractive index to one with a higher refractive index. Therefore, the medium pair of crown glass and ruby can be taken as an example, where light moves from the crown glass to the ruby.