Damage caused to human eyes by laser light is dependent on the frequency of the laser, as well as the exposure time. For red laser light, a typical value for the amount of energy per square meter that would cause damge is 10J/m2. How long could someone stare directly at this supermarket checkout laser before damage occurs

t = 3.14 10⁻² s

Explanation:

The typical lasers of the barcode readers are of the order of mW, which we look for the energy per unit of time

                 E = P t

The diameter of these lasers is of the order of a few centimeters

 The area of a circle is

               A = π r²

               A = π (1 10⁻²) = 3.14 10⁻⁶  m²

As the data they give is the intensity, we calculate with a rule of direct proportions the energy in the area of the laser reader

         10 j in 1 m²  what is the energy in 3.14 10⁻⁶ m²

       E = 10 J / m²   3.14 10⁻⁶ m²

       E = 3.14 10⁻⁵ J

Suppose the reader's power is 1 mW, let's calculate the time to reach the previously calculated power

       t = E / P

       t = 3.14 10⁻⁵ / 1 10⁻³

       t = 3.14 10⁻² s

So seeing these lasers directly causes vision damage very quickly.

the maxium height is 34m/s