1. A beam of light from a laser pointer passes through air and then enters a glass
block of refractive index 1.33. The wavelength of this laser beam in air is 490 nm.
What is the wavelength in the glass block?
a. greater than 490 nm
b. smaller than 490 nm
c. equal to 490 nm
d. answer cannot be determined from given information
The wavelength of the laser light in the glass block will be smaller than 490 nm, so the correct option is (b) smaller than 490 nm.
The Wavelength of 490 nm Light in Glass
The wavelength of 490 nm light in glass is a classic conceptual problem in optics that tests understanding of how light behaves when it passes from one medium to another, such as from air into a glass block. By using the relationship between refractive index, speed of light and wavelength, it becomes clear that the wavelength of light decreases inside a denser medium like glass, allowing students to confidently choose the correct MCQ option and strengthen their grip on refraction concepts.
Core Physics Concept
When light passes from one medium to another (here, from air into glass), its speed and wavelength change, but its frequency remains constant.
The refractive index n of a medium is related to wavelength by:
λmedium = λair/n
Calculation
- Wavelength in air: λair = 490 nm
- Refractive index of glass: n = 1.33
- Wavelength in glass: λglass = 490/1.33 ≈ 368 nm (approximately)
Since 368 nm is less than 490 nm, the wavelength in the glass is smaller than 490 nm.
Complete MCQ Solution
Question: A beam of light (laser) with wavelength 490 nm in air enters a glass block of refractive index 1.33. What happens to its wavelength in the glass?
Answer: λglass ≈ 368 nm
Option (b): smaller than 490 nm ✅
Glass has a refractive index higher than air, which makes the speed of light in glass smaller and thus its wavelength smaller (frequency stays constant). Using the formula gives about 368 nm, which is clearly less than 490 nm.
Option Analysis
Option (a): greater than 490 nm
This would mean the wavelength increases when light enters glass from air. However, for a medium with refractive index n > 1, the wavelength decreases according to λmedium = λair/n. Since n = 1.33 > 1, the wavelength must be smaller, not greater.
Option (c): equal to 490 nm
The wavelength would remain equal only if the refractive index of the second medium were effectively the same as that of air (≈1). Because the given refractive index is 1.33, there is a definite change in wavelength.
Option (d): answer cannot be determined from given information
The question provides both the wavelength in air and the refractive index of glass, which is sufficient to determine how the wavelength changes qualitatively (and even quantitatively if required). Therefore, the answer can be determined.
