CISCE Class VII Physics PDF

UNIVERSAL EDUCATION CISCE 2023-24 CLASS VII PHYSICS Ch. 4 - Light Energy Unit review page 44 1. Write true or false...

UNIVERSAL EDUCATION CISCE 2023-24 CLASS VII PHYSICS Ch. 4 - Light Energy Unit review page 44 1. Write true or false i The image formed by a plane mirror is virtual.- True ii We see things around us due to regular reflection of light.- False iii Rose appears green in light of all colours - False iv The primary colours are red green and yellow. - False 2. Fill in the blanks. i Angle of reflection = Angle of incidence ii The image formed by a plane mirror is virtual and same size of object iii A virtual image cannot be obtained on the screen. iv The colour of an opaque object is the colour of the light which it reflects 3. Answer the following questions i. How is the plane mirror made? Ans. A plane mirror is made using some highly reflecting and polished surface such as a silver or aluminium surface in a process called silvering. After silvering, a thin layer of red lead oxide is applied at the back of the mirror. ii. Explain the terms angle of incidence and reflected ray Ans. Angle of incidence An angle of incidence can be defined as the angle between a ray incident on a surface and the line perpendicular to the surface at the point of incidence. Reflected ray - Ray of light which goes back in the same medium after striking the surface is called reflected ray. iii. Draw a neat labelled diagram to show reflection of light ray from a plane m mirror  iv. Describe an experiment to verify the second law of reflection of light Ans. a drawing board, a white sheet of paper, a few common pins, a protractor, pencil, eraser and a plane mirror. Method: Fix the white sheet of paper firmly on the drawing board. Place the plane mirror on it and trace its outline on the paper. Then remove the mirror and draw the normal. Now place the mirror again on the outline. The normal will be reflected clearly on the mirror. Next place two pins in a straight line on one side of the normal on the white sheet of paper.Next place two pins on the other side of the normal in such a way that these two pins are in a straight line with the reflection of the two pins on the other side of the normal. Now remove the mirror and the pins and join the pin marks to the normal. Measure the angle of incidence and the angle of reflection. Both will be equal, proving the first law of reflection. Since the lines representing the normal, the incident ray and the reflected ray are all represented on the same sheet of paper, the second law is also verified. v. How do we see object around us. Ans. The bouncing of light into the same medium after striking is called reflection of light. We can see the objects around us due to reflection of light. When light from a luminous source fall on an object, a part of it is reflected back and enters our eyes. This is how we see objects around us. vi. Distinguish between regular and irregular reflection Ans. Regular Reflection Irregular Reflection This type of reflection occurs on This type of reflection occurs on smooth surfaces such as a plane rough surfaces such as a black mirror. board. In this type of reflection, the reflected In this type of reflection, the reflected rays are parallel to each other. rays are non-parallel to each other. In this type of reflection, the reflected In this type of reflection, the reflected rays are in one direction. rays are in different direction. vii. Define the term normal on a mirror Ans. A normal ray is a ray that is incident at 90 degrees to a surface. That is, the light ray is perpendicular or normal to the surface. viii. Describe the laws of reflection Ans. The three laws of reflection The angle between the incident ray and the normal is equal to the angle between the reflected ray and the normal. The incident ray, the normal and the reflected ray are all in the same plane. Incident ray and reflected ray are on different sides of the normal.  ix. What is meant by the term real image Ans. A real image is defined as one that is formed when rays of light actually meet or intersect at a point. A real image can be projected or seen on a screen. The best example of a real image is the one formed on a cinema screen. x. Name the primary and secondary colour Ans. Primary Colours Red, Blue, Green Secondary Colours- Yellow, Cyan, Magenta Pigment colours are different than light colours and their combinations may not give the same colour as discussed above. xi. Why are the certain objects opaque while certain others are transparent? Ans. Opaque substances do not allow the transmission of light. Any incident light gets reflected, absorbed or scattered. Light rays can penetrate the substance to some depth. Example black board, desk xii. State whether light slows down or speeds up in the following cases A. Light going from air to glass Slows down B. Light going from glass to water Speeds up C. Light going from water to air.- Speeds up