NCERT 7th Class (CBSE) Science: Light

Question: What do you means by lateral inversion? Answer: Lateral inversion means left looks like right and right looks like left in a mirror. Question: What is a spectrum? Answer: Spectrum is the band of colours seen when the white light passes through the prism and splits into seven colours. Question: What is Reflection? Answer: The bouncing off of light from a surface known as reflected light. Question: Name the mirror that forms only diminished image. Answer: Concave mirror. Question: What is bouncing off light from the surface called? Answer: Bouncing off light from the surface is called reflection of light. Question: A lens produces a magnified virtual image of an object. Name the lens. Answer: Convex lens. Question: What is the image that can be projected on a screen? Answer: Real image can be projected on a screen. Question: What is the splitting of light into its component colour called? Answer: Dispersion of light Question: What do you mean by rectilinear propagation of light? Answer: Rectilinear propagation of light means that beam of a light always travels in a straight line. Question: How can we change the direction of the light? Answer: We can change the direction of the light by the phenomenon called reflection. Question: What do you mean by reflection of light? Answer: When a ray of light falls on the surface of any mirror, it bounces back in the same medium. This is called reflection of light. Question: What is a mirror? Answer: Any polished or shining surface is called a mirror. Question: What do you mean by a real image? Answer: Image which cannot be obtained on a screen is called a real image. Question: What do you mean by a virtual image? Answer: Image which cannot be obtained on a screen called virtual image. Question: What is a convex mirror? Answer: A mirror which reflects the light from its inner polished side. Question: What is a concave mirror? Answer: A mirror, which reflects light from its outer polished surface. Question: What kind of image is formed by a plane mirror? Answer: It forms an erect, virtual and image of same size as that of the object. Question: What kind of image is formed by a concave mirror? Answer: It can form both real and virtual, erect or inverted image. Question: How will you identify a concave lens by touching it? Answer: A concave lens is thinner at the center and thicker at the edges. Question: What is rainbow? Answer: The rainbow is an arch of seven colours seen in the sky. Question: What does act as prism in the formation of a rainbow? Answer: Tiny raindrops suspended in atmosphere act as prisms in the formation of a rainbow. Question: What is the difference between a real image and a virtual image? Give one example of each type of image. Answer: Real image:The image which can be obtained on a screen is called a real image. In a cinema hall, we see the images of actors and actresses on the screen. So, the images formed on a cinema screen are real images. Virtual image: The images which cannot be obtained on a screen is called a virtual image. A virtual image can be seen only by looking into a mirror or a lens. The image of our face formed by a plane mirror cannot be obtained on a screen, it can be seen only by looking into the mirror. So, the image of our face in a plane mirror is a virtual image. Question: What is the principle axis of a concave mirror? Answer: A straight line passing through the two centers of curvature of the lens is called the principal axis of the lens. Question: What is principal focus of a convex lens? Answer: A parallel beam of light passing through a convex lens converges at a single point. This point is called the principal focus (F) of the convex lens. The distance between the principal focus and the optical centre of the convex lens is called the focal length of the lens. Question: Mention the characteristics of an image formed by a plane mirror. Answer: Characteristics of an image formed by a plane mirror are: The image formed by a plane mirror is always virtual and erect. The image undergoes lateral inversion. The size of the image formed by a plane mirror is the same as that of the object. The distance of the object from the mirror is equal to the distance of the image formed by the mirror (that is, image distance = object distance) Question: With the help of an experiment. Show that light cannot bend from its path. Answer: To demonstrate that light travels in a straight line or cannot bend from its path, we require three small cardboard sheets, three stands, candle and candle stand. Now make small holes at the centre of the three cardboard sheets. Fix the cardboard sheets on the three stands and arrange them in a straight line. Light the candle and place it in front of the hole made on the first cardboard sheet. See the candle flame through the hole of the third cardboard sheet. Now, move the third cardboard stand a little away from the line. We will observe that candle flame is visible when holes of all the cardboard sheets are in straight line. The light is not visible when third stand is moved away. This shows that light travels in a straight line. Question: Draw ray diagrams to represent the image formed by a convex lens and a concave lens when the object is kept between F and 2F. Answer: Question: Show diagrammatically how a convex lens produces a magnified and virtual image. Answer: Question: Distinguish between a concave and convex mirror. Answer: Concave mirror: It reflects light from its inner shining surface. It can form both real and virtual image. It can form both erect and inverted image. It can form a magnified image. Convex mirror: It reflects light from its outer shining surface. It always forms a virtual image. It always forms an erect image. It always forms a diminished image. Question: What happens when a sharp image of the sun is produced on a paper? Answer: By using a concave mirror, then after a few minutes, the paper starts at the point of sun's image and a hole is formed in it. This is so because the concave mirror converges a lot of sun's heat rays to a small point on paper. Question: State the uses of concave mirrors. Answer: Concave mirrors are used: As reflectors in torches, vehicle head lights and search lights to get powerful beams of light. As shaving mirrors to see a large image of the face. By dentists to see the large image of the teeth of the patients. In the field of solar energy to focus sun's rays for heating solar furnaces. Question: For driving a car what type of mirror would you prefer to see the traffic at your back? Answer: We prefer convex mirror for observing the traffic behind us because its field of view is much larger than the plane mirror. However, it gives erroneous idea about the speed of the vehicles behind us. But because of the first advantage, we still prefer it. Question: In a trick mirror, the image of head appear much smaller than that of the mouth and of nose. Explain why? Answer: A trick mirror is basically a concave mirror having small focal length. The head is comparatively away from the nose and mouth. So, the magnification of head is smaller than that of nose and mouth. This is the reason that nose and mouth appear to be large in size than the head. Question: How will you distinguish between a plane mirror, and a convex mirror without touching them? Answer: We can distinguish between these mirror just by bringing our face close to each mirror, turn by turn. A plane mirror will produce an image of the same size as our face and we will look our normal self. A concave mirror will produce a magnified image and our face will look much bigger. A convex mirror will look much smaller like a small child. Question: How will you show that a convex lens converges parallel rays of light to a single point? Answer: Place a piece of paper on the ground in bright sun light. Hold a convex lens some distance above the pieces of paper in such a way that a sharp image of the of the sun is formed on the piece of paper. Here the convex lens is converging the parallel rays of sunlight due to which the sun's rays get concentrated on a small part of the paper. The heat energy of focused sunlight rays burns the piece of paper. Question: Name type of mirror used by dentists. Why is it used for this purpose? Answer: Concave mirrors are used by dentists to see the large images of teeth of patients. Due to this, it becomes easier to locate the defect in the teeth. Question: Name the type of mirror which is used as a reflector inn the head lights of a car. Why is it used for this purpose? Answer: Concave mirror is used as a reflector in the head lights of a car. When a lighted bulb is placed at the focus of a concave reflector, then the concave reflector collects all the diverging rays of light of the bulb and converts them into a power beam of parallel rays. This parallel beam of light rays can reach a large distance in the darkness of night and make us see things up to considerable distances. Question: Define dispersion of white light and name the colours of white light in order. Answer: The splitting of while light into its constituent seven colours is called dispersion of white light. White light splits into the following colours: violet, indigo, blue, green, yellow, orange and red. Question: Why does dispersion of white light occur? Answer: light of all colours travel at the same speed in vacuum. But in any transparent medium such as glass, water, atc., the lights of different colours travel with different speed. The red light travels of all the seven colours. As a result the red light bends the least and the violet light bend the most. This unequal bending of the lights of different colours causes dispersion of white light. Question: What are the characteristics of images formed by a concave mirror? Answer: The characteristics of images formed by a concave mirror are: The image is not always the same size as the object. It can be larger or smaller depending on the distance of the object to the mirror. The image is formed on the screen. Such type of image that can be formed on a screen is called real image. The image is inverted when the object is kept far from the mirror. When it is brought very close to the concave mirror, an erect and enlarged image is formed. Question: What are the characteristics of images formed by a convex mirror? Answer: The characteristics of images formed by a convex mirror are: The image is erect. The image is smaller than the object. The image formed is virtual. Question: Write two uses of convex mirror. Answer: Convex mirror is used: As a year view mirror in vehicles to help the driver check the view of the traffic behind them. As a reflector in street lamps to spread light over a wider area. Question: The mirror outside the driver seat of a vehicle is usually a convex mirror. Printed on such a mirror is usually the warning, "Vehicles in this mirror are closer than they appear." What is the reason for this warning? Answer: The reason for this warning is that the Virtual image produced in a convex mirror is reduced in size and therefore looks smaller, just as distant object will appear in a plane mirror. A driver, thinking that the side view mirror is a plane mirror, might incorrectly deduce from the small size of the image that the vehicle behind is far enough away to ignore. Question: What is a spherical mirror? Distinguish between a concave mirror and a convex mirror. Answer: A spherical mirror is that mirror whose reflecting surface is the part of a hollow sphere of glass. Concave mirror: A concave mirror is the 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 the concave surface. The back side of concave mirror has a silver coating which is protected by a layer of red paint. The back side of concave mirror is represented by drawing short, oblique lines. Convex mirror: A convex mirror is that spherical mirror in which the reflection of light takes place at the convex surface (bulging out surface). The convex surface of this mirror is shown in diagram. The reflection of light takes place from the convex surface. The back side of a convex mirror has a silver coating which is protected by layer of red paint. The back side of convex mirror is represented by drawing short, oblique lines. Question: What is a rainbow? How is rainbow produced? What information do you get about sun light from the formation of a rainbow? Answer: Rainbow: The rainbow is an arch of seven colours seen in the sky. Just after rain, there are large number of small water drops suspended in the atmosphere. Each of these water drops acts like a small prism. As the white sunlight 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 formed in the sky called rainbow. The formation of rainbow having seven colours shows that white light consists of seven colours. Question: What is dispersion of light? Why does it take place? Answer: The phenomenon of splitting of white light into its seven component colours is called dispersion of light. Splitting of white light into its component colours is due to the fact that lights of different colours travel with different speeds. Question: Draw a diagram to show the dispersion of white light into seven colours by a glass prism. Answer: Diagram is as follows: Question: What is meant by spectrum? Name the colours of spectrum in their order. Answer: The band of seven colours dispersed by a glass prism is known as a spectrum. The seven colours in a spectrum are violet, indigo, blue, green, yellow, orange and red known VIBGYOR.

Class Notes NCERT Solutions for CBSE Students

NCERT 7th Class (CBSE) Science: Light

Question: What is a convex mirror?

Question: What is a concave mirror?

Question: What kind of image is formed by a plane mirror?

Question: What kind of image is formed by a concave mirror?

Question: How will you identify a concave lens by touching it?

Question: What is rainbow?

Question: What does act as prism in the formation of a rainbow?

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