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Science · 2025 · 5 marks
CBSE 2025 · Region 3 · Set 1 · Q36
(a) (i) "In refraction of light through a rectangular glass slab, the emergent ray is always parallel to the direction of the incident ray." Why?…
(i)
"In refraction of light through a rectangular glass slab, the emergent ray is always parallel to the direction of the incident ray." Why ? Explain with the help of a ray diagram. What happens when a ray of light falls normally on one of the faces of a rectangular glass prism ? Draw diagram.
(ii)
An object is placed at a distance of $\displaystyle 30$ cm from the optical centre of a concave lens of focal length $\displaystyle 20$ cm. Use Lens formula to determine the position of the image formed in this case.
(i)
A student wishes to study the image formation by a concave mirror using candle flame as object. State the type of the image formed by the mirror and mention the change in the image formed, if any, that he observes when the candle flame is gradually moved away from the pole of the mirror. Draw a ray diagram to show the image formation when the object distance is nearly equal to the radius of curvature of the mirror.
(ii)
A convex mirror used for rear-view on an automobile has a focal length of $\displaystyle 3$‧$\displaystyle 0$ m. If a bus is located at $\displaystyle 6.0$ m from this mirror, use mirror formula to find the position of the image of the bus as seen in the mirror.
Marking-scheme solution
(a)(i)
The extent of bending of the ray of light at the opposite parallel faces of the rectangular glass slab is equal and opposite. This is why the ray emerges parallel to the incident ray.
\[\begin{aligned}
\frac{1}{\mathrm{v}}= & \frac{1}{+3 \cdot 00 \mathrm{~m}}-\frac{1}{(-6 \cdot 00 \mathrm{~m})} \\
=\frac{1}{+3 \cdot 00 \mathrm{~m}}+\frac{1}{6 \cdot 00 \mathrm{~m}} & =\frac{2+1}{6 \mathrm{~m}} \\
\text { or } \mathrm{v} & =\frac{6}{3}=2 \cdot 0 \mathrm{~m}
\end{aligned}
\]
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CBSE Class 10 Science past-paper question from the 2025 board exam, with the answer as CBSE’s own marking scheme gives it. Where our answers come from.