CAPF AC 2021 Q19: Science & Technology – Physics
The following figure shows the image of a clock in a plane mirror:
Which one of the following is the correct time?
- (a) 2:35
- (b) 3:45
- (c) 9:15
- (d) 9:25
Answer: (d) 9:25
Explanation
Clock seen in a plane mirror
How to think: a plane mirror swaps left and right.
This is called lateral inversion.
So a hand pointing at 7 in the mirror really points at 5.
Shortcut: real time = 11:60 − time seen in the mirror.
(11:60 is just 12:00 written so that minutes can be subtracted.)
| Step | Working |
|---|---|
| Time seen in the mirror | 2:35 |
| Subtract from 11:60 | 11:60 − 2:35 |
| Hours | 11 − 2 = 9 |
| Minutes | 60 − 35 = 25 |
| Real time | 9:25 |
The options checked:
| Option | Verdict |
|---|---|
| (a) 2:35 | Incorrect; this is what the mirror shows |
| (b) 3:45 | Incorrect; its mirror image would read 8:15 |
| (c) 9:15 | Incorrect; its mirror image would read 2:45 |
| (d) 9:25 | Correct; its mirror image reads 2:35 |
Answer: (d) 9:25.
Why each option is right or wrong
- (a) Incorrect. 2:35 is the time read off the mirror image, not the real time.
- (b) Incorrect. A clock at 3:45 would look like 8:15 in the mirror.
- (c) Incorrect. A clock at 9:15 would look like 2:45 in the mirror.
- (d) CORRECT. 11:60 − 2:35 = 9:25, the real time on the clock.
Key point
Plane mirror gives lateral inversion. Real time = 11:60 − mirror time. Mirror 2:35 means real 9:25.
Remember
Mirror clock: subtract from 11:60.
Common mistakes
- Marking the time exactly as it appears in the mirror.
- Subtracting from 12:00 and getting the minutes wrong; use 11:60.
- Thinking a plane mirror turns the image upside down; it only swaps left and right.
Exam tip
Check by adding: real time + mirror time must make 12:00. Here 9:25 + 2:35 = 12:00.
Sources
- NCERT Class X Science — Light: Reflection and Refraction (image formed by a plane mirror)
- R. S. Aggarwal — A Modern Approach to Verbal and Non-Verbal Reasoning (Mirror Images)