1. Why the back of a CD or DVD shows rainbow colors
The rainbow colors on the back of a CD or DVD are not pigment colors. They are "structural colors" created when light diffracts (bends) through regularly spaced tracks (grooves) and interferes with itself [1]. The track spacing on a CD is 1.6 μm. Since 1 μm is one-thousandth of a millimeter, this means more than 600 tracks fit within a 1 mm width. However, educational measurement materials state that for Blu-ray (BD) discs, no rainbow is observed when white light hits them from the front [2]. Both are silver discs, so why is BD different? The answer appears in section 5.
2. How one spiral track on a CD adds up to about 5.4 km
CD data is arranged in a single track that spirals from the center outward [4]. With a track spacing of 1.6 μm and a data area radius of about 25 to 58 mm, the width of 33 mm divided by 1.6 μm equals approximately 20,000 turns. Adding the length of each turn results in a total length of about 5.4 km. All of this track fits into a spiral on one disc. A paper explaining CD specifications states the data spiral length is 5.38 km, which matches this calculation closely [4].
The tiny bumps and dips on the track are called "pits." Their width is about 0.5 μm and depth is about 0.1 μm [3]. If a human hair is 0.1 mm thick, these pits are about one-thousandth of that width. The length varies between 0.9 and 3.3 μm depending on the source, so treat all these numbers as "about."
3. How pits on a CD cancel light as steps instead of holes
A CD consists of four layers: a polycarbonate resin layer (1.1 mm thick), an aluminum reflection layer, a lacquer protective layer, and a printing layer. The laser passes through the transparent resin to hit the pits and lands (flat areas) on the reflection layer. From the reading side, the dips appear as bumps [1]. Light reflecting off a land returns directly. Light reflecting off a pit is shifted by about half a wavelength compared to the land's reflection. These two light waves interfere and cancel each other out, so the light returning from a pit is dark. The player detects the pits from these changes in brightness [3].
The pit depth creates this shift. The CD reading light has a wavelength of 780 nm, which becomes about 500 nm inside the resin. One-quarter of that is about 125 nm. The CD pit depth of 110 nm is roughly one-quarter of this wavelength. Because light travels through the step twice (going in and coming back), the optical path difference becomes about half a wavelength [3].
4. How a laser spot wider than a pit can still read the data
Even when focused by a lens, a laser spot cannot be made infinitely small due to the wave nature of light. This is the diffraction limit. The spot size is proportional to the wavelength and inversely proportional to the lens's numerical aperture (NA). NA indicates how strongly the lens can gather light [5]. As a rule of thumb, dividing wavelength by NA gives the spot size. Dividing wavelength by NA gives about 1.7 μm for a CD (780 nm and NA 0.45), about 1.1 μm for a DVD (650 nm and NA 0.6), and about 0.48 μm for a Blu-ray disc (405 nm and NA 0.85) [6]. Materials confirm the light diameter on the CD recording surface is focused to about 1.7 μm [3].
This 1.7 μm spot is wider than the 0.5 μm pit width or the 1.6 μm track spacing. The player does not read with a beam thinner than the pit. Instead, it reads features smaller than the focusing limit by detecting differences in brightness caused by interference [3].
5. How Blu-ray makes the light spot smaller with a shorter wavelength and stronger lens
Blu-ray reduces the light spot size by changing two factors compared to DVD. First, the wavelength is shortened from DVD's red 650 nm to blue-violet 405 nm. Second, the NA is increased from DVD's 0.6 to 0.85 [6]. Sony's explanation states this reduces the light spot area to about 1/5th of DVD's. The track spacing also shrinks to less than half of DVD's (from 0.74 μm to 0.32 μm). As a result, the capacity becomes about 25 GB, roughly five times DVD's 4.7 GB [7]. Calculating the area change shows the wavelength effect is about 2.6 times and the NA effect is about 2 times, totaling about 5 times, which matches the data. A simpler estimate appears in Canon's explanation: a blue laser has about half the wavelength of a red one, can make a spot of around 500 nm, and by simple calculation could pack four times the data onto the same disc [9]. Blu-ray's blue-violet 405 nm is not half of DVD's red 650 nm, however, and the actual capacity difference does not follow this simple calculation.
In 1996, Nichia Chemical succeeded in continuous room-temperature operation of a gallium-nitride-based blue-violet semiconductor laser (410 nm), and Blu-ray discs using this kind of laser were released in 2003 [8]. DVD capacity is about 7 times CD's, but the light spot area difference is only about 2.6 times. The reason for the remaining difference could not be confirmed in independent sources, so we do not state one.
Returning to the question from section 1: The BD track spacing of 0.32 μm is smaller than the wavelength of visible light (about 380 to 770 nm). Therefore, it does not meet the condition for light arriving from the front to diffract, so no rainbow is seen from the front. However, when viewed from an angle, colors appear in the order of blue, green, and orange [2]. The rainbow is a sign of fine grooves, but BD's grooves are too fine for a rainbow to be observed from the front with visible light.
6. How cover layer thickness and hard coatings affect scratches on discs
In Blu-ray, the recording layer is under a 0.1 mm protective layer, with a 1.1 mm substrate below it. DVD has 0.6 mm layers on both sides [5]. Sony explains that placing the recording layer closer to the lens reduces read/write errors when the disc tilts or warps [7]. However, the thin protective layer makes it vulnerable to scratches. Early BDs came in cartridges for protection. Companies developed hard coats resistant to scratches, making cartridges unnecessary, and they were discontinued [5].
For CDs, the recording layer is near the label side, about 0.1 mm from the surface. Sony notes that this is why CD-R labels warn against writing with pointed pens [7]. The protective layer thickness on the CD label side varies in sources from about 20 μm to 0.1 mm, so consider it "very thin."
7. How you can investigate disc rainbows at home
If you have discs at home, try observing them. Tilt a disc under room light and look at the rainbow colors.Three-band fluorescent lights are said to make the color separation easier to see [10]. Compare how the colors appear on a CD, a DVD, and a Blu-ray disc, and for the Blu-ray, look at it both from the front and at an angle [2].
SAFETY NOTE: Some light sources may have strong ultraviolet rays, so take care not to hurt your eyes. Do not reflect sunlight from a disc into anyone's eyes, and never look at the laser light inside a player.