Edexcel A-Level Physics Intensity of Radiation

Practise Edexcel A-Level Physics radiation intensity through I=P/A, inverse-square spreading, solar-cell power, photon rates and energy-transfer calculations.

Exam points

use I=P/A with surface area to calculate power, intensity or exposure time
apply spherical spreading area 4πr² to compare radiation intensity with distance
combine intensity with photon energy or time to find photon rate or incident energy

[Maximum number: 3]

A red light-emitting diode (LED) was placed into a circuit, as shown.

Figure for Question 2.3.44 - Intensity of radiation question 1 — Edexcel A-Level Physics AS

The LED emits light of wavelength of 627 nm and may be assumed to be 100% efficient. The battery has negligible internal resistance.

When the switch was closed, the reading on the ammeter was 9.2 mA.

The maximum intensity of sunlight at the Earth's surface is about $1100 \mathrm{Wm}^{-2}$ .

A student suggests that the intensity of light at a distance of 5.0 mm from an LED with a power of 0.69 W is greater than the intensity of sunlight at the Earth's surface.

Assess whether the student's suggestion is correct.

At $r=5.0\,\mathrm{mm}$ , $A=4\pi r^2=3.14\times10^{-4}\,\mathrm{m^2}$ . Therefore $I=P/A=0.69/(3.14\times10^{-4})=2.20\times10^3\,\mathrm{W\,m^{-2}}.$ This is greater than $1100\,\mathrm{W\,m^{-2}}$ , so the suggestion is correct.

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2.3.44 - Intensity of radiation question 1