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The number of photons emitted per second by a bulb of power 66 W emitting waves of wavelength 600 nm is ______. (Take h = 6.6×10⁻³⁴ J s.)

Asked in GUJCET 2023 · Counting photons

• (1) 2×10²²
• (2) 2×10¹⁹
• (3) 2×10²¹
• (4) 2×10²⁰

Step-by-step solution

Why the other options are wrong

• (1) Comes from converting 600 nm to 6×10⁻⁵ cm and then using that number as metres. Each photon then carries 100 times too little energy, so the count is 100 times too large.
• (2) Comes from writing 600 nm as 6×10⁻⁸ m instead of 6×10⁻⁷ m. Each photon then carries ten times too much energy, so the count is ten times too small.
• (3) Comes from writing 600 nm as 6×10⁻⁶ m instead of 6×10⁻⁷ m. Each photon then carries ten times too little energy, so the count is ten times too large.

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