UPSC Prelims 2008 Q133: Science & Technology – Physics
Assertion (A): In the visible spectrum of light, red light is more energetic than green light.
Reason (R): The wavelength of red light is more than that of green light.
- (a) Both A and R are individually true and R is the correct explanation of A
- (b) Both A and R are individually true but R is not the correct explanation of A
- (c) A is true but R is false
- (d) A is false but R is true
Answer: (d) A is false but R is true
Explanation
Checking an Assertion-Reason pair on light energy and wavelength
Assertion (A): Red light is more energetic than green light.
Reason (R): The wavelength of red light is more than that of green light.
| Statement | Verdict | Reason |
|---|---|---|
| Assertion (A) | False | It is actually the opposite: green light has MORE energy than red light, since energy is inversely related to wavelength |
| Reason (R) | True | Red light does have a longer wavelength than green light, within the visible spectrum |
Why A is false but R is true:
| Point | Detail |
|---|---|
| Energy-wavelength relationship | E = hc/λ — shorter wavelength means HIGHER energy |
| Red vs green wavelength | Red is longer, so red has LOWER energy, not higher |
Answer: (d) A is false but R is true.
Why each option is right or wrong
- (a) Incorrect. A is false (red light has LESS, not more, energy than green), so this option is wrong.
- (b) Incorrect. A is false, not true, so this option is also wrong.
- (c) Incorrect. A is false, not true, so this doesn't match either.
- (d) CORRECT. A is false (red light actually has LESS energy than green), while R is true (red light does have a longer wavelength than green).
Key point
Red light actually has LESS energy than green light (making the Assertion false), because energy is inversely related to wavelength — but red light's longer wavelength (the Reason) is indeed true.
Remember
Red light has LESS energy than green light (Assertion false) — but red's wavelength IS longer than green's (Reason true).
Common mistakes
- Assuming longer wavelength automatically means higher energy, when the relationship is actually inverse (E = hc/λ).
- Confusing the true wavelength comparison (red is longer) with an incorrect energy conclusion.
- Not applying the basic photon energy formula correctly.
Exam tip
Energy is INVERSELY proportional to wavelength (E=hc/λ). Longer wavelength (red) = LOWER energy, not higher.
Sources
- NCERT Class XII Physics — Dual Nature of Radiation and Matter (photon energy)
- NCERT Class XII Physics — Wave Optics (visible spectrum)