Light: Mirrors and Lenses MCQs (Class 8 Science, NCERT)
116 practice questions on Light: Mirrors and Lenses, a chapter in the NCERT Class 8 Science textbook, organised into 2 topics. Every question includes a written explanation. Pick a topic below, or try the samples first.
On walking from bright sunlight into a dim room, a person can see almost nothing at first, and things slowly become visible over the next minute. This happens because
Athe opening in the iris takes time to widen and let in more light
Bthe room really does grow brighter minute by minute
Cthe retina has been permanently damaged by the sunlight
Dthe eye lens turns into a mirror in the dark
Show answer and explanation
Correct answer: A - the opening in the iris takes time to widen and let in more light
In bright light the iris keeps its opening small, and it needs time to widen when the light suddenly falls, so at first too little light reaches the retina. Ordinary sunlight does not permanently damage the retina, and vision does return. The eye lens stays a lens, and the room itself does not change.
Q2
hard
A student examines a leaf with a magnifying glass and then slowly raises the lens away from the leaf. Beyond a certain height the leaf suddenly appears upside down. This shows that
Athe leaf itself has turned over on the table
Bthe eye stops seeing properly once the lens is held at that height
Cthe lens has changed into a concave lens as it was lifted, because it is now farther from the leaf
Da convex lens gives an enlarged upright image only while the object is close enough to it
Show answer and explanation
Correct answer: D - a convex lens gives an enlarged upright image only while the object is close enough to it
A convex lens gives a magnified erect virtual image only while the object stays close to it, and beyond that the lens starts forming a real inverted image instead. A lens cannot change its own type by being moved. The leaf lies still on the table, and the eye works normally throughout.
Q3
easy
For a plane mirror, the distance of the image behind the mirror is
Aalways fixed at $1\ \text{m}$
Bhalf the distance of the object
Cequal to the distance of the object in front of it
Ddouble the distance of the object, since light travels there and back
Show answer and explanation
Correct answer: C - equal to the distance of the object in front of it
The image distance always matches the object distance for a plane mirror, which is why the image seems to move step for step with you. Doubling or halving would make the image race ahead or lag behind, which is never observed. A fixed distance would keep the image still while the object moved.
Q4
easy
A concave lens is
Aof the same thickness at every point
Bthinner in the middle and thicker at the edges
Cthicker in the middle and thinner at the edges
Dalways thicker at one edge than at the other
Show answer and explanation
Correct answer: B - thinner in the middle and thicker at the edges
A concave lens is hollowed on both faces, so the middle is its thinnest part and the rim is thicker. Thicker in the middle and thinner at the edges describes a convex lens. Equal thickness everywhere describes a plain glass sheet, and a lens is not made lopsided from edge to edge.
Q5
medium
A convex mirror can never form an image on a screen because the image it forms is
Aalways inverted
Balways coloured differently from the object
Calways virtual
Dalways exactly the same size as the object
Show answer and explanation
Correct answer: C - always virtual
No light actually reaches the place behind the mirror where the image appears to be, so a screen there catches nothing. The image is erect, not inverted, and it is smaller than the object rather than the same size. Mirrors do not change the colour of the light they reflect.
Q6
medium
A convex lens gives a virtual, erect and enlarged image when the object is placed
Avery close to the lens
Bbehind the screen
Cvery far from the lens
Dexactly where the screen stands
Show answer and explanation
Correct answer: A - very close to the lens
Only when the object is close to a convex lens does it act as a magnifying glass and give an enlarged upright image. A distant object gives a real inverted image instead. The screen catches the image and is not a place for the object, so the last two options make no sense.
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