Class 7 Science Chapter 11 Important Questions

On identifying the size of the letters of newspaper, we can differentiate between the two types of lenses.
If image of letters is large or magnified, then the lens is convex lens and if the image of letters is smaller or diminished in size, than the lens is a concave lens.

In first case, we will use a concave mirror and a screen. As we know that a concave mirror forms a real image of a distant object at its focus, therefore, a real image of a distance tree will be formed on the screen placed at the focus of concave mirror.
In second case, we will use a convex lens and a screen. As convex lens forms a real image of a distant object at its focus, therefore, a real image of a distant tree will be formed on the screen placed at the focus of convex lens.

If on decreasing the distance between the object and lens, the size of the image increases, then the nature of the lens is convex type.
No, if the distance between object and lens is less than the focal length of the lens, then it will form a virtual image which cannot be obtained on a screen.

As we know that the side mirror of a scooter must be a convex mirror so that we can view a wide range of traffic spread over a large area.
But if plane mirror, is used, we will not be able to see large area of traffic behind us which may create a difficulty in driving vehicle and can cause accident.

Take three card boards of similar size and make a hole in the centre of each. Make sure that the size and position of the hole is the same in each cardboard.
Now put the three card boards in vertical position with the help of bricks in such a way that the holes in the three card boards are in a straight line. Place a lighted candle in front of the cardboard number 1. Look through the hole of the last cardboard number 3.
The flame can be seen clearly through the holes. When one of the cardboards is slightly raised, no flame can be seen. This shows that light travels in a straight line. No flame can be seen when the holes of the three card boards are not in a straight line because light can-not bend.

Take a circular cardboard disc of about 10 cm diameter. Divide this disc into seven segments. Paint each segment with the seven rainbow colours. Make a small hole at the centre of the disc. The disc loosely on the tip of a refill of a ball pen. Rotate the disc at a high speed. All the colours get mixed up and the disc will appear whitish. Such a disc is popularly known as Newton’s disc.

When light falls on the surface of an object, the object sends the light back. The process of sending back the rays of light which falls on the surface of an object is called reflection of light. The rays of light which falls on an object, is called incident ray. And the ray of light which is sent back by an object is called reflected ray. The object having polished, shining surfaces reflect more-light than objects having unpolished dull surfaces. In fact, any polished and shining surface can act as a mirror.

i) When an object is placed at a far-off distance from a convex lens, the image formed by convex lens is real, inverted and much smaller than the object.
ii) When an object is placed close to a convex lens or you can say at the less distance than its focal length, the image formed by convex lens is virtual, erect and larger than the object or magnified.

When we look into a mirror, we see our face. What we see in the mirror is actually a “reflection” of our face and it is called “image” of our face.
In this case, our face is the “object” and what we see in the mirror, is its “image”. Actually, when light from our face falls on the mirror, the mirror reflects this light. The reflected light seems to come out of the mirror and we see an image or a picture of our face in the mirror. There are two types of images that can be formed with light. They are “real images” and “virtual images”.

If we stand in front of a big plane mirror, we see the image of our body in it. Though our image appears to be just as we are, but there is the difference. This is because if we lift “our right hand” then our images lift its “left hand” and if we lift “our left hand” then the image appears to lift its “right hand”. This means that the right side of our body becomes the left side in the image, whereas the left side of our body becomes right side in the image. We say that the image formed in a plane mirror is laterally inverted. In other words, when an object is placed in front of a plane mirror then the right side of object appears to become the left side of image and the left side of image appears to become the right side of image. This change of sides of “an object” and its “mirror image” is called lateral inversion.

A spherical mirror is that mirror whose reflecting surface is the part of a hollow sphere of glass. A concave surface means “bent in” surface. A concave mirror is that spherical mirror in which the reflection of light takes place at the concave surface or bent in surface. The reflection of light takes place from this concave surface the back side of concave mirror has a silver coating which is protected by a layer of red paint.

Suppose a beam of parallel rays of light falls on a convex mirror from the left side. We will see that after reflection from convex mirror, the parallel rays of light are spreading out going away from one another. When the parallel rays of light spread out, we say that the rays of light are diverging.
The parallel rays of light diverge they cannot meet at a point in front of the convex mirror. The focus of a convex mirror is a point behind the mirror from which the parallel rays of light falling on convex mirror appear to diverge after reflection from convex mirror.
The distance from focus up to the convex mirror is called “focal length” of convex mirror. The focus of a convex mirror is behind it. The focus of a convex mirror is virtual because the light rays do not actually pass through it. Since, a convex mirror diverges a parallel beam of light rays, therefore, a convex mirror is known as diverging mirror.

The word “magnify” means “to make something look bigger” then, it actually is. A convex lens gives a magnified image of a closed object. Used in this way, a convex lens is commonly known as magnifying glass. A magnifying glass is a convex lens which is used to make small things look bigger or by producing their magnified images. To use a convex lens as a magnifying glass, the object to be viewed should be placed close to the convex lens so that a magnified and erect image of the small object can be seen on looking through the convex lens. In other words, the object should be at a less distance than the focal length of convex lens then we can see the larger view of the object.

Take a circular cardboard disc of about 10 cm diameter. Divide this disc into seven segments. Paint the seven rainbow colours on these segments. You can also paste, coloured papers on these segments. Make a small hole at the centre of the disc. Fix the disc loosely on the tip of a refill of a ball pen. Ensure that the disc rotates freely. Rotate the disc in the daylight. When the disc is rotated fast, the colours get mixed together and the disc appears to be whitish. Such a disc is popularly known as Newton’s disc.

Just after rain, there are a large number of small water drops suspended in the atmosphere. Each of these water drops act like a small prism. As a white sun light enters and leaves these tiny water drops, the various coloured rays of sunlight are deviated by different amounts and a long arch of seven colours called “rainbow” is formed in a sky.
The formation of rainbow having seven colours shows that white sunlight consists of seven colours. The seven colours are red, orange, yellow, green, blue, indigo and violet.