Physics · Electromagnetic Waves · NEET
Higher. UV wavelength (about 400 nm to 0.6 nm) is shorter than visible light (about 400 nm to 700 nm). Since c = f × λ, a shorter wavelength means a higher frequency, so UV has higher frequency and more energy per photon than visible light. In the full spectrum the order by increasing frequency is: radio, microwave, infrared, visible, ultraviolet, X-rays, gamma rays. NEET loves testing this order.
NCERT states UV radiation is absorbed by ordinary glass. So the UV part of sunlight is blocked by the glass and never reaches your skin. Tanning happens because UV makes the skin produce more melanin; with no UV getting through, there is no tanning behind glass. This same fact is why welders use special glass goggles to block the strong UV from welding arcs.
NCERT gives about 4 × 10^-7 m (400 nm) down to 6 × 10^-10 m (0.6 nm). The upper limit (400 nm) is where visible violet ends, and the lower limit (0.6 nm) is where X-rays begin. There is no sharp line between UV and its neighbours; the boundaries are approximate and based on how the waves are produced and detected.
The Sun is a strong UV source, but most solar UV is absorbed by the ozone layer at about 40 to 50 km altitude, protecting living things. Large amounts of UV are harmful to humans. That is why depletion of the ozone layer by chlorofluorocarbons (CFCs like freon) is a serious global concern. This link between UV, ozone and CFCs is a common one-line NEET fact.
UV purifies water. UV radiation is germicidal — it kills germs — so UV lamps are used in water purifiers. Treating cancer cells is done by gamma rays, not UV. Do not mix these up: in the 2026 matching PYQ, UV pairs with 'purifying water' and gamma rays pair with 'treating cancer cells'.
Match List-I (electromagnetic wave) with List-II (major application) and choose the correct option. List-I: P. Microwave, Q. UV rays, R. Gamma rays, S. Radio wave. List-II: I. For purifying the water, II. For warming the food, III. For AM and FM communication systems, IV. For treating the cancer cells.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
UV rays are electromagnetic waves with wavelengths from about 400 nm to 0.6 nm, lying between visible light and X-rays. They have higher frequency and more energy than visible light.
The Sun is the main natural source (most solar UV is absorbed by the ozone layer). Man-made sources are special UV lamps, very hot bodies, and welding arcs.
Killing germs in water purifiers (UV is germicidal), high-precision applications like LASIK eye surgery, and detecting invisible writing and forgery. UV lamps also help study fine structure of molecules.
Welding arcs produce large amounts of UV. Since ordinary glass absorbs UV, welders wear special glass goggles or masks to protect their eyes from this strong UV radiation.
Yes, in large amounts UV is harmful to humans. It causes tanning and sunburn by triggering extra melanin. Excess exposure can damage skin and eyes, which is why the protective ozone layer is important.
