UPSC Prelims 2009 Q118: Science & Technology – Physics
Which one among the following has the highest energy ?
- (a) Blue light
- (b) Green light
- (c) Red light
- (d) Yellow light
Answer: (a) Blue light
Explanation
Which colour of visible light has the highest energy
Among the visible spectrum, a specific colour's light carries the most energy per photon.
| Option | Verdict | Reason |
|---|---|---|
| (a) Blue light | Correct | Blue light has a shorter wavelength and higher frequency than green, yellow, or red light, giving it the highest energy among these four |
| (b) Green light | Incorrect | Has less energy than blue light |
| (c) Red light | Incorrect | Has the longest wavelength, hence the lowest energy among these four |
| (d) Yellow light | Incorrect | Has less energy than blue light |
Energy and wavelength relationship:
| Point | Detail |
|---|---|
| Energy formula | E = hc/λ — shorter wavelength = higher energy |
| Order (high to low energy) | Blue > Green > Yellow > Red |
Answer: (a) Blue light.
Why each option is right or wrong
- (a) CORRECT. Blue light, having the shortest wavelength among these four, carries the highest energy.
- (b) Incorrect. Green light has lower energy than blue.
- (c) Incorrect. Red light has the lowest energy among these four, due to its longest wavelength.
- (d) Incorrect. Yellow light also has lower energy than blue.
Key point
Among blue, green, red, and yellow light, BLUE LIGHT has the highest energy, since its shorter wavelength corresponds to higher photon energy (E = hc/λ).
Remember
Among blue, green, red, and yellow light, blue light has the highest energy.
Common mistakes
- Forgetting the inverse relationship between wavelength and energy (shorter wavelength = higher energy).
- Assuming red light, being at one end of the visible spectrum, has higher energy than blue.
- Not knowing the basic energy ordering across visible-light colours.
Exam tip
Shorter wavelength = higher energy (E=hc/λ). Blue light's short wavelength makes it the highest-energy option among these four colours.
Sources
- NCERT Class XII Physics — Dual Nature of Radiation and Matter (photon energy)
- NCERT Class XII Physics — Wave Optics (visible spectrum)