∙BIO Laser Safety A 0.75 -mW laser emits a narrow beam of...

$\bullet$ BIO Laser Safety A 0.75 -mW laser emits a narrow beam of light that enters the eye, as shown in FlGURE 25 -41. (a) How much energy is absorbed by the eye in 0.2 s? (b) The eye focuses this beam to a tiny spot on the retina, perhaps 5.0$\mu \mathrm{m}$ in diameter. What is the average intensity of light $\left($ in $\mathrm{W} / \mathrm{cm}^{2}\right)$ at this spot? (c) Damage to the retina can occur if the average intensity of light exceeds $1.0 \times 10^{-2} \mathrm{W} / \mathrm{cm}^{2} .$ By what factor has the intensity of this laser beam exceeded the safe value?

$\bullet$ BIO Laser Safety A 0.75 -mW laser
emits a narrow beam of light that
enters the eye, as shown in FlGURE 25 -41. (a) How much
energy is absorbed
by the eye in 0.2 s? (b) The eye focuses this beam to a tiny spot on
the retina, perhaps 5.0$\mu \mathrm{m}$ in diameter. What is the average intensity of light $\left($ in $\mathrm{W} / \mathrm{cm}^{2}\right)$ at this spot? (c) Damage to the retina can
occur if the average intensity of light exceeds $1.0 \times 10^{-2} \mathrm{W} / \mathrm{cm}^{2} .$
By what factor has the intensity of this laser beam exceeded the
safe value?

Key Concepts

Safety Threshold Exposure

Safety thresholds establish upper limits for exposure to certain intensities of light to prevent damage, such as retinal injury. Understanding the comparison between the actual intensity delivered by a laser and a set safety threshold is crucial in evaluating potential hazards and ensuring compliance with safety standards.

Laser Power and Energy

Laser power is a measure of the energy output per unit time from a laser. In physics, the energy delivered by a laser can be calculated by multiplying its power by the duration of exposure. This concept is essential in understanding how much energy an object (or in this case, the eye) absorbs over a given time period.

Intensity and Area

Intensity is defined as the power per unit area and is expressed in watts per square meter (or per square centimeter) when converted appropriately. Calculating the intensity requires knowledge of the beam’s power and the area over which the energy is distributed, emphasizing the importance of area measurement in assessing the energy density of the beam.

Optical Focusing and Spot Size

Optical focusing involves concentrating a beam of light to a smaller area, which increases its intensity. The spot size, often given as the diameter of the focused beam, is critical in determining the area over which the laser’s power is distributed and therefore directly influences the intensity of light at that focal point.

Transcript

00:01 Okay, so in this problem we have a laser beam with a power.

00:05 Let's put it here, power of the laser beam of 0 .75 milliwats.

00:16 The eye of this person, the laser beam is focused on the eye of this person, and the eye focused the laser on a spot with a diameter.

00:27 Let's put in here, diameter 1 of 5.

00:32 Micrometers and the first item of this problem let's put in here item a we have to calculate the energy absorb by this i during a time let's put in here a 2 .2 seconds that's correct let's see 0 .2 seconds so so to calculate the energy, we just need to remember that energy is defined as the power, multiply by the time.

01:12 So we have the power, we have the time, this is just going to be 0 .75, the multiplies 0 .2.

01:24 Therefore we have an energy absorb of 0 .15 millijoules.

01:32 That's the answer to the first item.

01:36 The second item.

01:38 The second item, we already calculate the energy.

01:41 We have to calculate the intensity of the laser.

01:46 So intensity is defined as the power divided by the area.

01:55 So we want to know the intensity of the laser being on this spot, the focus point.

02:02 So we have here the power which is 0 .75.

02:10 Let's put in watts now times 10 to the power of minus 3 watts...