# Simple Optics Q: Green Filter + Blue Light = ??

• Pencil123
In summary, the conversation discussed a question on an exam about a green filter and blue light. The speaker's teacher stated that the filter should appear black, but the speaker believes it should appear blue. The conversation then explained that for an ideal green filter, the blue light would not pass through, but for a real filter, some blue light may pass through. This is because the pass band for a green filter is wider and allows more light to pass, whereas a narrower band would result in a dimmer appearance. Additionally, the conversation mentioned that mixing blue and yellow pigments can result in a green filter that appears green to the eye, but would still block blue light. This is due to the spectral response of the mixture.
Pencil123
There was a question on my exam. There is a green filter and blue light is directed to the filter. Normally It should pass through the blue light but my teacher says that we wouldn't be able to see it so the color we see would be black but I think it would be blue. What is the answer?

Usually, when people talks about [color] filter, such as green filter, then they mean a filter which visually appears [color]. So in your case, green filter should appear green. This means this filter only let through frequencies corresponding to green colors and block other frequencies.colors.

For an ideal green filter, with just a narrow band pass, the blue light will no go through.
For a real, usual filter (like the ones used in photography), some of the blue will go through.

This is not as straightforward as it seems at first. When you buy a filter, specified as green, it will pass a range of wavelengths in the region of 'spectral' green. It's pass band will be quite wide in order that the light coming through will be bright. Your eyes will see about 1/3 of white sunlight through the filter. (If it let only a narrow band it would look very dim)
Blue wavelengths would probably lie outside the pass band so they wouldn't get through.

Here's an added confusion which may explain the OP's dilemma. Mixing blue and yellow paint (pigments) will produce a pigment that looks green. If you make a filter this way, it will look green when white light passes through. Blue light will be blocked by the yellow pigment, even though the blue pigment will pass it. The shape of the spectral response of this mixture will probably be very 'sloppy' and might pass a bit of blue but not much. Look up Subtractive Colour Mixing.

## 1. What is a green filter and how does it work?

A green filter is a type of optical filter that selectively transmits green light while blocking other wavelengths of light. It works by absorbing or reflecting all other colors except green, which is allowed to pass through.

## 2. Can a green filter be used to block blue light?

Yes, a green filter can be used to block blue light to some extent. While it is designed to transmit green light, it can also absorb some blue light due to the overlapping wavelengths of these two colors.

## 3. What happens when a green filter is combined with blue light?

When a green filter is combined with blue light, the green filter will allow green light to pass through while blocking most of the blue light. The resulting color will appear to be mostly green, with some blue light still present.

## 4. How does a green filter affect the intensity of blue light?

A green filter will decrease the intensity of blue light by blocking some of it. The amount of blue light that is transmitted through the filter will depend on the specific characteristics of the filter, such as its thickness and the materials used.

## 5. Are there any practical applications of a green filter and blue light combination?

Yes, there are several practical applications of a green filter and blue light combination. For example, green filters are commonly used in photography to improve contrast and reduce glare, while blue light is often used in medical treatments for conditions such as jaundice.

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