Gr 8 Sub Science Ch 12 Light W.B. Work

 

1.	Fill in the blanks.
	a.		The retina is made up of light sensitive ___________ and _________cells.
	Ans.		The retina is made up of light sensitive rod and cone cells.
	b.		A kaleidoscope works on the principle of ________________ reflections.
	Ans.		A kaleidoscope works on the principle of multiple reflections.
	c.		When a parallel beam of light is reflected in different directions by a surface, it is called ______________ reflection.
	Ans.		When a parallel beam of light is reflected in different directions by a surface, it is called diffused reflection.
	d.		The image of an object in the eye is formed on the _____________.
	Ans.		The image of an object in the eye is formed on the retina.
2.	Match column ‘A’ with column ‘B’.
	Column A					Column B
	a.		Cataract		i.		Lengthening of the eyeball
	b.		Myopia		ii.		Shortening of the eyeball
	c.		Night blindness		iii.		Age related defect
	d.		Hypermetropia		iv.		Deficiency of vitamin A
	Ans.		a – iii                b – i                c – iv                d - ii
3.	Name the following.
	a.	A device which works on reflection of light.
	Ans.	Periscope/ kaleidoscope
	b.	The point inside the human eye where the image is not visible.
	Ans.	Blind- spot
	c.	The phenomenon which enables us to see cartoon movies in a cinema hall.
	Ans.	Persistence of vision
	d.	The type of image of an object formed in a plane mirror.
	Ans.	Virtual and erect
4.	Answer the following questions in one word or one sentence.
	a.	What type of lens is used to correct the defect of near sightedness?
	Ans.	Concave lens
	b.	Define dispersion.
	Ans.	Dispersion is splitting of white light into its constituent colours.
	c.	Does light gets reflected or refracted to produce the spectrum?
	Ans.	Refracted
5.	Answer the following questions in 2-3 sentences.
	b.			What is lateral inversion?
	Ans.			The phenomenon due to which left-hand side of an object appears as the right-hand side of the image and vice versa in a plane mirror is called lateral inversion.
	c.			If two plane mirrors are placed making an angle of 30 degrees, how many images of an object will be formed?
	Ans.			Number of images
				= 12-1
				= 11
	Picture study
	The diagram represents a defect in eyesight. Observe the diagram and answer the following questions.
	a.	Identify the defect.
	Ans.		Hypermetropia
	b.		What is the cause of this defect?
	Ans.		Shortening of eyeball
	c.		What can we correct this defect?
	Ans.		By using convex lens
			diagram with the corrective lens.
	Challenge Time
	Observe the images and answer the following questions.
	a.		Identify the image showing irregular reflection.
	Ans.		Image (b) shows irregular reflection
	b.		Are the laws of reflection followed in irregular reflection. Take the help of a diagram to explain.
	Ans.		Laws of reflection are valid for all kinds of reflection, be it regular or irregular. In case of irregular reflection of parallel rays, the reflected rays are not parallel. If we consider the individual rays, we see that the laws of reflection apply to each of them. The laws of reflection require the angle of incidence and the angle of reflection to be the same and for the incident ray, the normal at the point of incidence and the reflected ray to all lie in the same plane. These two laws of reflection will be obeyed in a diffused reflection.
	Practical learning
	While running, something entered into Sahil’s eye. He immediately washed his eye with cold water. But as the irritation persisted, he immediately reported to his class teacher. His class teacher took him to an eye doctor.
	·         What should be done if some foreign particles enter your eyes?
	Ans.		We must do the following things: ·         Restrict eye movement ·         Do not rub eyes ·         Try to clean it with splash of clear water ·         Contact eye doctor immediately.
	·         Why it is not advised to rub your eyes when there is an eye irritation due to foreign particles?
	Ans.		Foreign object may cause scratches on our cornea or rubbing. It may also cause bleeding of cornea.
	·         Do you think Sahil was right in his action?
	Ans.		Yes.
	·         What value of Sahil is seen here?
	Ans.		He is intelligent, knowledgeable and aware of such kind of small accidents.
6.	HOTS
	a.		Why is the sky blue in colour? Research and discuss it in the class.
	Ans.		·         The sky looks blue because of a process called scattering. ·         Sunlight has many colours, but blue light has shorter waves. When sunlight passes through the air, the tiny particles in the atmosphere scatter the blue light in all directions. This is why we see the sky as blue during the day.