Question

Which of the following is a dimensionless quantity?

• A. Magnetic flux density
• B. Electric flux density
• C. Lumen flux density
• D. Optical density

Answer 1

Answer: (D)

Optical density

Answer 2

Optical density is the ratio of the speed of light in two media. Whenever light passes through an optically dense medium, it bends towards the normal. The extent to which it bends is measured as the optical density of the medium. As optical density is the ratio of two same physical quantities , therefore it has no unit. Hence,**optical density is a dimensionless quantity. **

The other 3 quantities mentioned in the options above have a specific unit and thus have dimensions.

• Magnetic Flux Density - It is the force acting per unit current per unit length on a wire placed under magnetic field. Its SI unit is Tesla.
• Electric Flux Density - It is the measure of the intensity of electric field generated by an electric charge. Its SI unit is NC-1.
• Lumen Flux Density - It is a measure of the intensity of light passing through a unit section per unit time. Luminous flux is measured in Lumens.

Also Read:Ray Optics & Optical Instruments